pull requests are broken

I have a repository X, and a fork of it called X-beta. I'm trying to create a pull request from X-beta into X, but the "merge" button is disabled in the resulting PR, and there's an error that says that "some or all changesets in this pull request have been removed".

see https://bitbucket.org/LanceCorrimal/dolphinviewer3/pull-request/2/trying-to-pull-into-release