Tainted aquagoblin doesn't spawn the Corrupted aquagoblin
 

I pulled the tainted aquagoblin twice today.

I fought him in the water once, on the island above the second time, and in neither cases did he spawn the corrupted aquagoblin when the tainted died.

This is a bug, right?

no it's not a bug, killing the tainted has a CHANCE % to spawn corrupted, not 100 %... some are higher then others. This shaman one is rare

So... there are a few that are not 100%.

AquaGoblin has about a 50% spawn rate
Barracuda has about a 50% spawn rate
Gorilla has 100% spawn rate from the live tainted, but the undead model tainted is a 0% spawn rate.
Everything else is 100% spawn rate. Seahorse, Seafury, Brownie, Hill Giant, Mammoth.