Skeletons and wither skeletons can spawn anywhere in the Nether

Wither skeletons have a certain percentage chance of spawning a normal skeleton instead. Intended.

What does

/locate Fortress

return? This location (or nearby)?

I can confirm this. I've attached the output of /locate Fortress

It's already marked as fixed for the next snapshot.

Aw man, this wasn't intended? I did find it difficult at low level, but it was also challenging. Good to know.