I have this model: https://prnt.sc/gsgs2s
I hit "TposeMesh" and pose my model using some brushes and Tranpose tools: https://prnt.sc/gsgrns
Then I go back I press "Tpose > SubT" and I got this: https://prnt.sc/gsgr0x
This isn't always happening. This problem also occurs Tposing the mesh and then exporting(with groups enabled!) to maya and back to zbrush.
When I do this(zbrush to maya and back) without groups enabled i don't have problems.
I also get this message when I import back to zbrush from maya: https://prnt.sc/gst5ow , because i get the same problem when using maya to pose my mesh and when just pose the character in zbrush, they may help you underestand what is happening.
Also i experiment a bit and when i delete some polygons from the mesh and then Tpose > SubT i got another message so this isn't causing the problem. I also tried meshing up my polygroups and making a real mesh with moving vertices all over the place and i didn't get the problem so this isn't the case too.