Regarding the thread you have linked to, I am not getting any such error…the grub on boot up is showing the entries to my newest fedora34 gnome + win 10 properly, no issues. My problem is that it is not showing entries to the fed34 cinnamon which exists in the hard disk.
As far as I know all Fedora spins use the same location on the EFI partition: /boot/efi/EFI/fedora. You can check with efibootmgr -v. It will most likely show only one “Fedora” entry.
So every time a Fedora spin is installed it writes to the same EFI path as the other ones. And running grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg overwrites what another spin has written.
I don’t run multiple Fedora spins anymore. But I have an EFI system (Thinkpad T14) and install multiple Linux distributions (Fedora, Endeavour, OpenSUSE, etc.). What helps me a lot is using Refind as a boot manager. Install Refind and allow it to be the default in the EFI boot order. It detects the other Linux’s EFI entries and kernels and lets you pick which one to boot.
Just in case you also might run into it:
Lately I also had troubles with grub2-mkconfig not working correctly (not creating all menuentry lines in grub.cfg). I found the fix for that is to set GRUB_ENABLE_BLSCFG=false in /etc/default/grub. Should you decide to use Refind you can also disable GRUB from trying to detect other OS: GRUB_DISABLE_OS_PROBER="true".
Optionally, you could forgo grub altogether. I’ve used rEFId and rEFIt in the past with great results, but with my current setup I’m dual booting with two physical drives and my BIOS/EFI is handling boot menu duties.