Thanks for the hint. I found the error.

To test the update functionality originally, I additionally used --source-overrides to pin a certain lock file, which referenced (on purpose) an older commit in the oe-core wrynose branch. Then, I assumed, by calling bitbake-setup update this would then sync the layer to the latest upstream HEAD (fast-forward from the older commit to the latest). However, it seems that the bitbake-setup update does not work in that combination (if the revs are initially pinned). Which is fine for me.

Thanks for the fast help!