This post of mine - https://hexbear.net/post/971624?scrollToComments=false - has a 1 comment but I can't see it. There's nothing there.
I can see a comment from HexReplyBot there:
It's probably the hexreplybot. I have no idea how the bot works but I assume it automatically doesn't send a comment to the OP but the messages still appear in the header anyway.
There's no way for the bot to actually do that. The only thing the bot can do is basically send some text to the Lemmy server and then Lemmy makes that appear as a comment for everyone. There's no way to not send a comment but make it appear as a notification. What you're seeing seems like a Lemmy bug.
You can open an issue on Lemmy's issue tracker: https://github.com/LemmyNet/lemmy/issues