My observed result in de.wikpedia (1.28.0-wmf.10) was, that the page is actually saved, but neither the edit mode nor the save dialog is removed.

It should take you back to the edit summary page in case there is an error and then immediately saves the page. If it didn't save the page maybe there was an error. Does anything appear in the console?

Yeah, it did take me back to the Edit summary page but didn't save the page . Not getting any error in the console.

Could be T140574, but that produces an error in the console.

Is this still broken?

It is only broken on test2, but working fine everywhere else for me.

In T140410#2499989, @Ryasmeen wrote:

It is only broken on test2, but working fine everywhere else for me.

For me when I did https://test2.wikipedia.org/w/index.php?title=Foobar&diff=294790&oldid=285122 it switched back and then saves, which is the expected outcome…