I can also confirm this from upgrading from 1.12.2 to 1.13.1. The problem seems to be that existing nether fortress bounding boxes have disappeared, causing normal nether mobs to spawn rather than nether fortress ones. For example, I have only seen zombie pigmen and ghasts spawn in my large wither skeleton farm. Note that the ghast spawning mechanic prohibits them from spawning in nether fortresses (1), so this supports my claim of the disappearance of the bounding box.
Newly generated fortresses are unaffected and behave as expected.
1. This mechanic may have changed in 1.13.x in regard to observations I have made.
Synchronous Times
I can also confirm this from upgrading from 1.12.2 to 1.13.1. The problem seems to be that existing nether fortress bounding boxes have disappeared, causing normal nether mobs to spawn rather than nether fortress ones. For example, I have only seen zombie pigmen and ghasts spawn in my large wither skeleton farm. Note that the ghast spawning mechanic prohibits them from spawning in nether fortresses (1), so this supports my claim of the disappearance of the bounding box.
Newly generated fortresses are unaffected and behave as expected.
1. This mechanic may have changed in 1.13.x in regard to observations I have made.