Apparently one of the lemmy.ml admins was overzealous in banning all User-Agent strings that contained the word "bot". Bans were entered for all of the individual strings containing that word which were observed in their webserver logs, which impacted kbin's reported agent of "kbinBot".

The issue has been fixed, and I observed that one of my kbin posts to a lemmy.ml community was successfully pushed to the original instance.

Edit:

Here are all the links that I've found with the lemmy.ml admins discussing the issue:

• https://lemmy.world/comment/1082149 (appears to have been where they first noticed and fixed the issue)
• https://lemmy.ml/post/1920972 (posted a few hours later, provides additional context for what happened)