Never had something similar to what you are experiencing.
Possible medium reaction with that particular plastic...?
Generally, It is good to do a test, for example:
If I was in your position
Id look for answers online, but if unsuccessful in finding exact answer,
Id take one (sacrificial) guy and put a clear matt varnish over the sticky surface to see if it will hold up.
If the test holds up, then I would apply 1 or 2 more coats on top for good luck.
After priming my Reaper bones minis, they get sticky, sometimes that Reaper white plastic reacts in sticky way to the contrast paints too. So, I just paint over the sticky surface normally with acrylic paints, and then seal it with a matt clear varnish. That works for me on sticky Reaper Bones.
Hope you can find the exact answer that you need.