That was my first idea as well. But I was locking for a smarter solution. :)
I discovered a strange conflict in raspian 64x. Before I did always use the 32x variant.
I did some research and testing why the display_hdmi_rotate command in the config.txt apparently didn’t work. It seems there is a conflict with DRM VC4 V3D driver (which is enabled by default). Just un-commenting the line #dtoverlay=vc4-kms-v3d the screen rotates (even the splash picture) as expected. The driver obviously overwrites the display_hdmi_rotate command.
Now the MM and splash have the right orientation. Only the default weather app says loading, but it will be replaced by another module anyhow.
Thanks, andyyy