Today I installed something then removed it and Windows disappeared from the GRUB menu!
There was a warning when I uninstalled glfw-x11. I think I actually installed glfw-wayland and it brought several components along but never showed up as installed so, in my innocence, I thought that I must have installed glfw-x11 by mistake and decided to removed it.
When I tried to change to Windows later on I rebooted my machine and... it wasn't there! Confused, I went back to Linux and searched for this issue. Naturally the issues aren't usually the same but the some mentioned the sudo update-grub command. Upon running it it warned that it would ignore OSes detected by the os-probercommand. That was unexpected so I ran lsblk to check if my partitions were fine and everything looked okay, then I ran gparted to check if the drive had the right flags (boot and ets) and...
I saw a warning sign next to the partition! Its tooltip told me that it couldn't open that partition without mtools. I installed it by using the graphical Package Manager but running sudo update-grub still failed. Apparently the system was smart enough to enable the GRUB_DISANLE_OS_PROBER flag but I'd have to disable it myself so I edited /etc/default/grub and added the line GRUB_DISANLE_OS_PROBER=false, saved and ran sudo update-grub one last time. Finally the warning was gone and after rebooting I could switch to Windows. While unexpected it was easy to fix and teaches us to pay more atention when uninstalling stuff.