Tablet's Map To Monitor button should be greyed out in relative mode
When a tablet is in Absolute mode, the cursor position always represents the stylus position on the tablet.
When a tablet is set to Relative mode, it works like a mouse, i.e. three downward strokes with lifting in between will push the cursor further down - just as a mouse would do.
In Relative mode, the Map To Monitor setting doesn't make sense, just as it doesn't make sense for a mouse either. The button should be greyed out. Arguably the current monitor mapping (if any) should reset too but it's probably better to leave it and ignore the mapping in the mutter code instead.