Editing PR updates other PR with same target branch

Steps to reproduce:

1. Imagine a repo with branches A, B, and C.
2. Raise a PR from branch A to branch B, call it "A to B".
3. Raise a PR from branch A to branch C, call it "A to C".
4. Edit the second PR as follows:
   1. change the name from "A to C" to "I'm updated"
   2. change the target branch from C to B
   3. change the description to "XXX"
   4. click "Update pull request"
5. The update appears to succeed, in that there's no error message, but:
   1. the PR you edited has not changed at all (the title is still "A to C", and the target branch is still C)
   2. the first PR (which you didn't edit) now has:
      1. the title "I'm updated" (which should be the title of the second PR)
      2. the description "XXX" (which should be the description of the second PR)