When I left click anywhere in the 3D viewer my vertex groups disappear! I redownloaded Blender and tried a few projects - same result!?
This only happened in 2.93.4 so I opened in 3.0.0 which was all okay until I realised the groups disappear if I open in older versions (not backward compatible). I would work entirely in 3.0.0 but it crashed when I tried to render motion blur in previous project.