Check settings of the solid booster engines then the core engine changes from hydrolox to kerolox for some reason.
I was able to reproduce this bug reliably, but no other players were able to.
~~Edit: I restarted the game and I can not reproduce the bug anymore. I am confused. It is probably a unique case.~~
Edit 2: I played around a bit, flew and undo and exited flight, and went on Discord for a while and the bug is occurring again.