Mobs not spawning on old chunk structures

Been to a Monument and a Fortress in survival and no structure-based mobs are spawning on pre-loaded old chunk structures. I conquered a monument yesterday and there were no Guardians at all. No Guardians but there were Elder Guardians that I fought. Same is happening in the Nether Fortress. No Wither Skeletons are spawning and Blaze only spawn from the spawners. Ghast can spawn in the Fortress as well. Please fix this because I can't be bothered to look for another Fortress or Monument to make farms.