Create pull request between hg bookmarks: There are no changes to be pulled

In my repository I have an @ bookmark as well as small bookmarks for feature development: feature/a and feature/b. I want to create a PR from feature/a into the @ bookmark.

I am able to select the correct bookmarks in the "create new pull request" form.
"diff" and "commits" tabs are showing the correct changesets, but after selecting "Create pull request", I am getting this error:

There are no changes to be pulled