Thanks a lot for the pointers! I had a deeper look and that is apparently completely expected behavior that barebox uses the internal device tree for booting the kernel as long as global.bootm.oftree is not set.
So I somehow misconfigured barebox so that global.bootm.oftree was not set. I just expected that it fails then and does not silently boot the internal device tree. Entirely my fault, but maybe someone runs into a similar problem in the future.
Thanks,
Florian