Envelopes in the XM format have a resolution of 64, just like in almost all other module formats. MilkyTracker may show a higher resolution, but in the end it has to divide all values by 4 when saving them to XM. This is similar to OpenMPT showing a pan range from 0 to 256 in many places even for formats that only support a pan range from 0 to 64 - it does this for consistency, but in the end when saving the pan value, it has to divide it. Milky is doing the same with envelope values.

FWIW I will keep this open but not because of XM (as explained), but because in general this would be good to have in a redesigned MPTM format at some point in time.