I believe that’s a bug in ‘qt-build-system’: like ‘gnu-build-system’, it
should pass #:graft? #f (patch below).  Failing that, it’ll end up using
a different #:qtbase depending on whether or not grafts are enabled.

Does that make sense?

I found a similar issue in ‘python-build-system’ in
<https://issues.guix.gnu.org/58579>.

Thanks,
Ludo’.