Ok, so after much experimentation, I discovered this.
When it happens to the female thralls, their voice often gets blanked out along with boobs set to 1.0. When it happens to male thralls, their voice doesn’t seem to reset, but their endowment setting also is set to 1.0 along with breast size.
I loaded an old save to screenshot correct settings, loading most recent game save with the issue and corrected settings, but they wouldn’t save! I was quitting, Exiting to main menu and loading back from there, but the settings would be incorrect!
So after correcting the settings, I tried quitting game completely, instead.
After launching the game and loading, the settings remained corrected.