We found a bug that grub is not able to reasonably handle two Fedora installations sharing the same /boot/efi partition. Reason is that the paths in RPMs for grub are hard-coded so the new system will break booting of the previous system. This will not happen with other combinations like Fedora/RHEL etc... However, it will still happen when system have Fedora 41 and you are installing Fedora 42 next to it.

This issue was raised by this bug which was closed.

We are not able to resolve this issue, however, we should warn user to make the installation which would break existing installation. IMHO (jkonecny) we should only warn user because there might be a way how to make this working manually after the installation so I wouldn't hard block users from this but they should be aware of the potential problem after installation.