Wither skeletons do not spawn with either the latest beta or stable build.

I tried it on a nether fortress on a test server with only this plugin. No wither skeletons after flying through two fortresses.

Disabled the plugin and killed all entities and it didn't even take 2 minutes to find several wither skeletons.

If you need more information, let me know.

Did you try killing all entities and then rechecking before disabling the plugin?

Yes I did that twice and nothing showed up...

Ok, try with antigrinder disabled.

I can pretty savely confirm that this option changes the behavior.
With "Inhibit Monster Grinders: false" they spawn just fine and with it set to true nothing seems to spawn.

Did you possibly forget nether bricks as a "natural spawn block" or something?

Hope that helps.

Yea, I was looking at that. Thanks for testing.

Btw, as mentioned on spigot I never did say that this plugin doesn't mess with spawning mechanics. Just that more tests had to be conducted to verify.

Nether bricks are included in natural spawn blocks for the nether. Hmm...

Hmm, there is NETHER_BRICK and NETHER_BRICKS...