In 3.21.1, and 3.21.2 a change to a shortcut looses all previous shortcut changes.
This does not become apparent in the current session; a modified shortcut works fine together with all the previous modifications, until you close QCAD.
But-
When you re-open QCAD, the only customised shortcut available is the last one that was changed, every other customised keyboard shortcut has gone back to default.

Windows 10 64 , Gigabyte laptop i7 processor