Because of the current state of this forum regarding replying to old posts, I made a new post.
Lately I raised an issue about this problem in 'chenall/grubutils' on Github (Issue #12)
Thanks to mrexodia this issue is closed in Development Build 2023-04-01 (by a1ive)
Some action pictures on grub4dos AND grub4efi:
BTW also fat mkfile and fat copy do not have related problems anymore.