You can try to remove the install-grub package or even blacklist any update to grub related packages.
We made install-grub part of the default installation one year ago, so it actually updates grub in efi and mbr. However if you dual boot it may be not good.
See if changes actually block your system to boot, made by this newer grub.
Arch never cares about grub. There most people install and configure it once.
I have SDDM but don’t use it because it has become very buggy with Wayland. I use greetd for a year instead. I’m afraid none of these can have any effect logically, because none changed when this happened.
If other packages working together with these packages changes then yes, the issue could be linked to it even if not originated by the above packages. Maybe there is an incompatibility.
OK, the new version of grub — or perhaps install-grub — was to blame. By uninstalling install-grub, the system became like < 2024, i.e., it stayed with a working grub version (which is 2.12.r350.g0e367796 from the stable branch for me). Now I’m in an up-to-date Manjaro Testing without a problem.
That being said, IMHO, it makes perfect sense to worry about future updates of Manjaro Stable, exactly because of this issue.