Yes, that is fixable without reinstalling Windows, but you know this forum is for Manjaro, not Windows. So ask that in a Windows Forum of your choice or use a search engine.
Fair. Thank you very much for pointing me to it @megavolt
Found the issue. Funny thing is tho (maybe interesting for others):
Windows does not show the Hybrid-MBR. It shows, that all is running perfectly with GPT.
Google for mbr2gpt there is a Microsoft Learn Site for using it. Conversion takes about 5 seconds.
Afterwards sudo update-grub to let it detect the “new” Windows entry.