Looking at the code in the desktop interface, we mount the host system /usr/share/fonts with the bind flag rather than rbind (aka recursive bind). This means that the the host system /usr/share/fonts directory will be available in the sandbox’s mount namespace, but none of the mounts from below that directory.
You should be able to inspect the sandbox mount namespace using snap run --shell gimp, and then see what’s visible in the fonts directory. It would also be worth running sudo /usr/lib/snapd/snap-discard-ns gimp to see if snapd has become confused trying to update an existing mount namespace. It would make sense to try snap-discard-ns on both systems: it may not make things work, but it will hopefully get both computers to give the same result.
If it does look like the problem is use of bind instead of rbind, we can probably fix that. I didn’t consciously choose not to make it recursive.
As for solutions that will work with the current version of snapd, a bind mount into your home directory under either ~/.local/share/fonts or ~/.fonts should work. It will only make the fonts available to the current user, but that’s probably all you need.