Probably a #Kbin #bug, probably not.
What I did:
1. Created a new Kbin “Thread” in a #Guppe group (or Kbin “Magazine”)
The new Kbin Thread was received by Guppe and pushed to its members.
2. Using a Kbin account, I replied to that Kbin Thread.
The reply was not seen by Guppe and thus no post/reply was pushed.
3. Using a non-Kbin account, I replied to that Kbin Thread.
Kbin itself received the reply and displays it as a Thread comment.
4. Using a Kbin account, I replied to the non-Kbin reply (see #3).
Guppe received it and the reply was pushed to Guppe members.
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So… it seems that a Kbin account replying to a Kbin Thread is not being received by Guppe (or Kbin is not pushing it).
Note: I have not tested if the Kbin “Magazine” is not from another instance. The steps would be slightly different as the non-Kbin account first have to subscribe/follo that “Magazine“.
Here is the test thread: https://fedia.io/m/test@a.gup.pe/t/969/Testing-how-posting-a-Kbin-thread-works