Bias in random parts with mutagenic buildup on PM submods

[lelodu87]

Describe the bug

Crocodilia scales skin is overrepresented in PME when a pawn has mutagenic buildup: they're very likely to have this mutation.

To Reproduce

• Have a minimal mod-list like on one of these pictures:
  
  
• Spawn some random colonists/use your default colonists.
• Give them mutagenic buildup in one of these ways: centrifuge, explosion or slurry puddle. I did not try with the rain or weapons, but it should work. Each time I stopped with a moderate amount of mutagenic buildup. Let us note that I did not notice the problem with mutonite and chaomorph partials.
• After some time (half a day in game, a day could be better), crocodilia scales tend to appear on a majority of pawns. For me, around 15 out of 20 had them. They also tend to have high crocodilia %.

Expected behavior

Crocodilia scales skin should be very rare (only for crocodile morph + alligator morph, out of a dozen base game skins). Out of 20 pawns, perhaps 1 or 2 max should have it.

Additional context

Overall, this behaviour seems to happen both for PME and PMI for any part added in them. There seems to be a bias in benefits of the submod in the random mutation selection while having mutagenic buildup.

[lelodu87]

Seemingly, shuffling the list of mutations does not change anything. Also there seems to be no visible bias (aka higher chance number) when throwing a random number to know whether a mutation is applied or not.