I have no definite proof yet, just want to share experience in case someone else notices this. I had a sim going in 2.37.5 with mutations enabled. When 2.37.6 came out, I saved the sim and loaded it into the new version. Mutants did not appear anymore. So I checked the mutations panel and found that mutations were set to "Disabled". I toggled them back into "Enabled", and it's been 100,000 cycles and I still don't see any mutants. The "Average mutations" grpah agrees with me. I checked about 10 bots and all of them say "No mutations".
Looks like a bug, smells like a bug, but I am not 100% sure.