Have you tried removing them from your z-wave network to see if it occurs as a standalone thermostat?
I have 2 of these plus a Trane Z_wave thermostat which is virtually the same, and I do not have this issue. I also have an M1G but I use Homeseer for control.
Are they switching in minutes, hours, or days? Always at the same time (together)? Always a fixed time from when they were changed back? Is there someone in your house who does not understand how to operate the thermostat?
Regards,
Vector