First of all @spacecase-25. None of our developers uses FDE at all. Hence we also DONT test this at all from our end. If the community want to encrypt their drives, simply go for it. You made that decision and you’re in the driver seat.
Regarding grub: Arch doesn’t maintain grub updates as it should be normally. You have to have some monolithic approach to have the binaries shipped by the grub package and the installation of grub in MBR or UEFI in-sync. Manjaro and all the other deviates are using Ubuntu’s update-grub script, which only updates the menu but not grub itself. As Arch already stated, it is really hard to know how you installed grub to begin with. You can read my indepth statement on this matter if you want.
Stable branch never means that you get fully tested packages and if you update you don’t have to worry that your system may break. Do a web search. Manjaro is not the only distro which may have that error. As soon as you encrypt your installation there must be some sort of understanding on how to get back to that data when some went wrong.
The Manjaro user community is on the problem and will provide a guideline on how to solve the issue if you have the issue. If your system boots, well fine. If you want to get rid of the “nag screen” even better. Not every installation is the same so we can’t do some simple magic to “fix” something which is not broken.