Mobs specific to generated structures such as the Nether Fortress and Guardian Temple are no longer spawning. If there were any mobs present when the chunk the structure was in unloaded they will be present when it is reloaded but if killed no new mobs will be spawned. 

Bug appeared directly after 19w08a and continued into 19w08b

Attempted adjusting difficulty level to no result, attempted reloading chunks to no result.