Again, your doing something strangely if you have in fact made this mutation, look at the tab labeled Austere on this link to the wiki here, and you'll see that it says they can't be bred unless in the nether or with alveary heaters: http://ftbwiki.org/Bee_Species Also, the point to the mutator is obvious, I have enough nether stars to easily breed any species with a 100% chance whenever I want, if I so choose.I can guarantee I've mutated austere in a vanilla desert biome using an apiary and nothing more than three soul frames - and the frames are a maybe, most base forestry breeds I didn't bother.
In this current game, I've never made a thermionic fabricator, much less alveary heaters.
In my prior game I'm fairly certain I used the same setup in a vanilla extreme hills (with Both_2 tolerance) but wont swear I didn't travel to a desert. I do know that I got 100% of the breeds available at the time other than dyes without ever making a mutator, and frankly don't see the point of them.
Sinister and Fiendish do require the nether, and one or the other is required for Frugal, but neither Frugal nor Austere require the nether to mutate. Or even a desert.[DOUBLEPOST=1365866863][/DOUBLEPOST]
Btw, if you were correct here (you're not) then it wouldn't be a biome restriction. Fiendish will not mutate outside of the nether no matter how many heaters you use.