BUG - Actions with `on: push` limited to a branch will trigger on that branch being merged into another

Apologies if this is a duplicate, couldn’t find anything similar, though I would think someone else would have come across this before. Might be intended function and I’m just not seeing the logic.

I have a workflow that uses this (and no other on):

on:
  push:
    branches:
      - master

It works great. However, a coworker tried to effectively rebase by merging master into their feature branch. For some reason when they opened their PR (again, merging the master branch into their branch) the workflow kicked off. There is another workflow that fires on PR open that also fired normally, but I would not expect a PR to fire an on: push for the branch being merged from, only the branch that was merged into - and only then when the merge was executed.

This is in a private repo, happy to share with GH admins privately. Thanks!

I agree that with the above trigger it doesn’t make sense that the workflow was triggered merely by merging the master branch into a feature branch. You may want to contact support about that. I also saw something strange in a specific repo the other day, where an on: push workflow got triggered in the upstream repo even though there was only a push to a fork. It’s concerning even if it’s not a widespread problem and perhaps an implementation bug.