I have played thousands of hours since EA and never recall seeing this ever before Patch 8. It can also be triggered by Respec and using Magic Mirror. It's actually quite annoying. It also affects Hierlings and companions made using the Companion Mod wand
Good to hear that I'm not imagining things - other than the Minsc-glitch (which I've seen from the game with all sorts of patches/versions) I had never seen this bug in any of my runs.
Weirdly enough (and I'm keeping my fingers *very* crossed), after seemingly happening all the time in the very early stage of the run, it hasn't happened in quite a while now. The last time it *did* happen was the instance I described earlier during the first respec-session with Withers (party level 3). Though I'll admit that I'm kinda reluctant to go near Withers with SH for fear that she'll glitch out again instantly - she seems the most susceptible to this sort of thing, at least in my play-through.
Party is now at level 6 with ~80% of Act I done and neither natural level ups nor the occasional respec (only did one for Karlach to fix her attributes and one with Gale to change his feat) resulted in glitched portraits.
Again: Since I don't swap characters around a whole lot at this stage (other than rotating Lae'zel and Karlach from day to day), neglected characters like Wyll or Astarion *might* be glitching out without me noticing it.