OLD: When I am walking through a nether fortress, no wither skeletons or normal skeletons spawning. Also found no blazes in the fortress. The blaze spawners are fine. Also no Pigmen spawning in side. Only a few on the bridges. (No longer true)

NEW:Latest version of Spigot there are more mobs spawning on normal. And every type is now spawning. But still less then a vanilla server.

When I tested this on a 1.13 vanilla server within 1 minute of walking around I found 2 wither skeletons and 2 normal skeletons. Where after 15 minutes on spigot I got 0 zero spawning. (Only 2 magma cubes and 3 zombie pigmen in the whole fortress.

Used a copy of the spigot world. Diffucilty lvl is normal. In the new testing I used a up to date spigot version(30-7-2018) and a new generated world.

Edit:

Seed: 3494217688620033322 (different from original bug report)

Location: x:384 z:-368 (use /locate Fortress for quick in chat teleport command)