Im making this post because I found almost no mention of this phenomenon online. I was struggling with creating polygroups on my large model with many polygroups and several subdivisions. Seemingly at random, my clean/straight polygroup edges would become jagged.
This happened inconsistently when I saved, when I rotated, when I left a mask, and possibly others.
I could not figure out what the cause of this was and it was driving me crazy. It turns out that if you model has subdivisions and polygroups, while performing various actions, Zbrush switches through the subdivisions of your model. So while your polygroups are smooth at your high poly, they are chunky at your midpoly. To solve this I deleted my low polys because I was working on frozen subdivision levels anyway.
Hi @Alex_Weber
When multiple subdivision levels are involved the polygroups must be created at the lowest level of subdivison. Polygroups created at higher levels of subdivison may not cleanly translate to the lower poly geometry resulting in the shifting borders when changing subD levels.
For example you know that a polygroup of a single polygon will in most cases cleanly subdivide into 4 new polygons when defined at the lower level. If the polygons are defined at higher level they may contain polygons split from multiple source polygons. The program will have to make guesses about where they go, resulting in the borders shifting on subd switch.
