[poppler] Anyone knows why some gitlab branches don't let me rebase on the webui?

[Albert Astals Cid]

In 
  https://gitlab.freedesktop.org/poppler/poppler/merge_requests/37
there is a "Rebase" button i can press

but in 
	https://gitlab.freedesktop.org/poppler/poppler/merge_requests/28 
there is not.

But if i rebase manually using git commands, it can be rebased, so it's not like it has conflicts or anything.

Anyone has any idea why?

Cheers,
  Albert