I have been using UGS for a while no problems.
Recently I carved a repeat item, so it is a known g-code in MM, and when it finished the DRO was displaying in imperial.
I have checked $13 and that is 0.
I will uninstall UGS and reinstall hopefully that will clear it.
This is NOT a problem that stops the machine working as I enter a MM number and it moves the correct amount, by vernier, but displays in imperial.
G20 in the gcode?