Unrelated branches message for PR to commit's parent

We have two branches:

feature/something (current commit: b4c8910c9497) and release (current commit: a6fbf21f3a46)

Where commit a6fbf21f3a46 is the parent of b4c8910c9497. When attempting to create a PR from the feature to the release branch we get a message: b4c8910c9497 & a6fbf21f3a46 are unrelated.

How can this be, the second is literally the parent of the first. They can't be any more related...

Three things that might be relevant:

1. The parent commit is a merge from a previous PR
2. The feature branch is in a forked repository (also on Bitbucket cloud)
3. It has worked before, so it's not a privilege issue