Hello 
Discourse ForumHelperBot helps me to investigate the issue. 
Publication type: Full Topic
Post object type: Note
Topic first post federates correctly to Mastodon 
Reply posts are delivered but NOT shown in followers’ timeline 
Root cause found in logs - reply posts have incorrect addressing:
"cc": ["https://.../ap/actor/GROUP_ACTOR_ID"]
Should be:
"cc": ["https://.../ap/actor/GROUP_ACTOR_ID/followers"]
Thanks