@julian Hi. Your reply won't have made it back, because of this issue - NodeBB is putting the wrong actor in the activity.
It's not a PieFed issue, pretty much all other federated platforms will reject activities if it's wrong. For example, on Lemmy, this post never made it there: https://lemmy.ml/c/activitypub@community.nodebb.org?dataType=Post&sort=New
OP's post was Announced by NodeBB as this:
{
  "@context": "https://www.w3.org/ns/activitystreams",
  "actor": "https://piefed.social/u/GroupNebula563",
  "cc": [
    "https://piefed.social/u/GroupNebula563",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "https://community.nodebb.org/post/https%3A%2F%2Fpiefed.social%2Fpost%2F468827#activity/announce/1739242064700",
  "object": {
    "@context": [
      "https://www.w3.org/ns/activitystreams",
      "https://w3id.org/security/v1"
    ],
    "actor": "https://piefed.social/u/GroupNebula563",
    "audience": "https://community.nodebb.org/category/30",
    "cc": [],
    "id": "https://piefed.social/activities/create/ZBBVVw3sp3wAdZ3",
    "object": {
      "attachment": [],
      "attributedTo": "https://piefed.social/u/GroupNebula563",
      "audience": "https://community.nodebb.org/category/30",
      "cc": [],
      "commentsEnabled": true,
      "content": "<p>PieFed is very similar to Lemmy, but with a couple small differences that a couple people prefer. It se                                                                                                                  ems to be retrieving the community okay, we'll see if this post goes through.</p>\n",
      "id": "https://piefed.social/post/468827",
      "language": {
        "identifier": "en",
        "name": "English"
      },
      "mediaType": "text/html",
      "name": "Test from PieFed!",
      "nsfl": false,
      "published": "2025-02-11T02:47:42.609106+00:00",
      "sensitive": false,
      "source": {
        "content": "PieFed is very similar to Lemmy, but with a couple small differences that a couple people prefer. It see                                                                                                                  ms to be retrieving the community okay, we'll see if this post goes through.",
        "mediaType": "text/markdown"
      },
      "stickied": false,
      "tag": [],
      "to": [
        "https://community.nodebb.org/category/30",
        "https://www.w3.org/ns/activitystreams#Public"
      ],
      "type": "Page"
    },
    "to": [
      "https://community.nodebb.org/category/30",
      "https://www.w3.org/ns/activitystreams#Public"
    ],
    "type": "Create"
  },
  "to": [
    "https://community.nodebb.org/category/30/followers"
  ],
  "type": "Announce"
}
The first 'actor' is wrong - it shouldn't be https://piefed.social/u/GroupNebula563, because you don't have the privkey to Announce anything as that user, and for them to send the activity to https://community.nodebb.org/category/30/followers doesn't make sense. The first 'actor' should be https://community.nodebb.org/category/30, which it was before a recent change.
Likewise, your reply was sent out as:
{
  "@context": "https://www.w3.org/ns/activitystreams",
  "actor": "https://community.nodebb.org/uid/2",
  "cc": [
    "https://community.nodebb.org/uid/2",
    "https://www.w3.org/ns/activitystreams#Public"
  ],
  "id": "https://community.nodebb.org/post/103212#activity/announce/1739244715728",
  "object": {
    "actor": "https://community.nodebb.org/uid/2",
    "cc": [
      "https://community.nodebb.org/uid/2/followers",
      "https://piefed.social/u/GroupNebula563"
    ],
    "id": "https://community.nodebb.org/post/103212#activity/create/1739244715677",
    "object": {
      "@context": [
        "https://www.w3.org/ns/activitystreams",
        {
          "Emoji": "toot:Emoji",
          "toot": "http://joinmastodon.org/ns#"
        }
      ],
      "attachment": [],
      "attributedTo": "https://community.nodebb.org/uid/2",
      "audience": "https://community.nodebb.org/category/30",
      "cc": [
        "https://community.nodebb.org/uid/2/followers",
        "https://piefed.social/u/GroupNebula563"
      ],
      "content": "<p><a href="\"https://piefed.social/u/GroupNebula563\"">@groupnebula563@piefed.social</a> the message was received! Hello! :smile:</p>\n",
      "context": "https://community.nodebb.org/topic/07a57511-3b8b-4efd-a496-88824f72dbaa",
      "id": "https://community.nodebb.org/post/103212",
      "inReplyTo": "https://piefed.social/post/468827",
      "name": "Re: Test from PieFed!",
      "published": "2025-02-11T03:31:54.966Z",
      "replies": "https://community.nodebb.org/post/103212/replies",
      "source": {
        "content": "[[@groupnebula563@piefed.social](https://community.nodebb.org/user/groupnebula563%40piefed.social)](https://community.nodebb.org/user/groupnebula563%40piefed.social) the message was received! Hello! :smile:",
        "mediaType": "text/markdown"
      },
      "summary": null,
      "tag": [
        {
          "href": "https://piefed.social/u/GroupNebula563",
          "name": "[@groupnebula563@piefed.social](https://community.nodebb.org/user/groupnebula563%40piefed.social)",
          "type": "Mention"
        },
        {
          "icon": {
            "mediaType": "image/png",
            "type": "Image",
            "url": "https://community.nodebb.org/assets/plugins/nodebb-plugin-emoji/emoji/android/1f604.png?v=70fb7184b74"
          },
          "id": "https://community.nodebb.org/assets/plugins/nodebb-plugin-emoji/emoji/android/1f604.png?v=70fb7184b74",
          "name": ":smile:",
          "type": "Emoji"
        }
      ],
      "to": [
        "https://www.w3.org/ns/activitystreams#Public",
        "https://piefed.social/u/GroupNebula563"
      ],
      "type": "Note",
      "updated": null,
      "url": "https://community.nodebb.org/post/103212"
    },
    "to": [
      "https://www.w3.org/ns/activitystreams#Public",
      "https://piefed.social/u/GroupNebula563"
    ],
    "type": "Create"
  },
  "to": [
    "https://community.nodebb.org/category/30/followers"
  ],
  "type": "Announce"
}
- the Announce actor should again be 
https://community.nodebb.org/category/30, because whilst you do have the privkey for https://community.nodebb.org/uid/2 it's not you that's Announcing this, it's the category. In Mastodon terms, an Announce is a Boost, so this activity is saying that you're Boosting your own post, which also doesn't make sense.