sys-apps/portage-2.2.10 check destination RO on "install" phase - before preinst & prerm. It can cause some troubles in packages, mounting own destinations (grub, but I trap in my overlay), as "install" still sandboxed to it job. Anymore: good idea to move this check after prerm (& preinst) - it from common sense of "prerm" & "preinst" phases - prepare destination, so destination is "ready" only after it.
Another idea might be to fix the packages that mount things on their own, or at least make this behaviour non-default and non-silent.
Mounting like eg grub:2 does is at best fragile and can unwittingly expose users to security issues and data loss. Consider the case where /boot is on a usb drive not normally connected when the system is runnint, using a /dev/sd* entry in fstab; then the user (or someone else) connects a different usb drive.