moderator: It does seem quite a few have issues, but it's not consistently reproducible. Sorry about the problems. An issue was opened, but for the foreseeable future here're some workarounds that should help:
Please like the linked-to workaround that worked for you, and leave a comment below if further explanation is needed.
@bijanbwb, continuing the discussion from Mastering Github:
As one can see in the picture below, the incorrect submission alert message reads:
We found the webhook, but it's triggered by Pull Request activity.
I believe it should read instead:
We found the webhook, but it's not triggered by Pull Request activity.
Also, after changing the webhook and resubmitting the answer it was still failing to accept, had to delete and create a new webhook from scratch instead.