Yes, as I tried EndeavourOS they converted to systemd boot. which I found was really nice.
This might be the issue that was with Grub.
But I noticed (see How to Double Check I am on Most Stable System?) I have installed and uninstalled kernels and surprisingly everything went fine. I had to do nothing manually to update Grub.
It is clear Manjaro developers are doing a great job in this regards.
Generally, I prefer to stick to the defaults as much as I can. But at the same time I play a lot with my system! I hope I won;t break it.