Newly created MR: `Merge blocked: the source branch must be rebased onto the target branch`

Steps:

• open a MR like: fdroid/fdroiddata!12467 (merged) or fdroid/fdroiddata!12484 (merged)
• try to merge

Expected behaviour:

• be able to merge

Actual behaviour:

• Merge blocked: the source branch must be rebased onto the target branch

Odd thing:

• there's nothing to rebase, the tree is unchanged between the moment the MR was opened and when I'd like to merge

Fix:

• wait for someone else to change the tree (or change it yourself by committing directly)
• then rebase
• then you can merge

Might be related: #213608 (closed)

This too: #369378 (closed)