@jmorganmkm I found this item in the bug fix list in version 3.4.0 at https://github.com/Duet3D/RepRapFirmware/wiki/Changelog-RRF-3.x#reprapfirmware-340 (search for CNC to find it):
In CNC and laser mode it is permitted to omit G0/G1 at the start of a line of GCode and just provide parameters, provided that the previous command was G0 or G1. However, only parameter letters that were present in that previous G0/G1 command were recognised.
So this is a bug in 3.3 which is fixed in 3.4.