Hi,
Wow that was quick. Thanks a lot. :)
Yes a field for player group would be perfect.
Well I'm not sure how to send you my Bestiary XML but I imported "CorePlusUnearthedArcana.xml" from here. After I ran into problems, I deleted the Gavelsoft folder from Users\*\AppData\LocalLow and my installation folder and tried importing to a clean install -> same problem. So the only thing that was present before importing was the preinstalled stuff.
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I just checked in 1.5.6 and the issue is still present.