I have the same problem on my switch version:
Before Nehter update: 1 oder 2 wither skeleton troops spawn in 5 minutes
Now: I kill one wither skeleton, 1 troop spawns behind me.
My Nether fortress is completely filled with skeletons and blazes, even the roof is full of them. I can add Screenshots later.
Similar problem on Nintendo Switch.
Underground base with torches on wall and light sources on ceiling
Before nether update: no mobs
After nether update, dark spots on ground and base filled with skeletons and creeper. Torches on ground fixed it.
Couldn't find anything on the internet about light changes.