I'm using a two-step workaround for now:
1. Select 'MGRS' as the output coordinate system and pull out only the '48PUV' part
2. Run again and select 'UTM' as the output coordinate system. The resulting value in the 'UTM' field looks like:
48 N 377299 1483035
Take the final 5 digits of each of the two numbers at the end and append them on to the '48PUV', giving you the MGRS coordinate of:
48PUV7729983035
Buyer beware / test before using / etc. Quick site for comparing coordinates in testing:
https://www.movable-type.co.uk/scripts/latlong-utm-mgrs.html