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!