Mob behaviour and spawning issues

In both of my servers (one Vanilla 1.15.2, the other one FML 1.15.2) mob behaviour and especially spawning are severely broken. Crepeers explode even if I run 5-6 blocks away from them, endermen don't teleport, zombies don't take knockback, spiders are stuck in corners and attack me in plain daylight, if it hit a Zombie Pigman all the zombies in a 100 block radius endlessly swarm me (about 20 per wave) ... On the other hand, an absurd amount of monsters spawn, I'm talking about packs of 3 creepers (mind you I'm playing on easy), packs of 3 endermen, 4 skeletons and zombies, pigmen everywhere. But the most broken thing are light levels. When checking on the debug menu even though I'm clearly in an enlightened area (above 7) the debug shows a lighting of 4-5, causing monsters to spawn in corners of my house and on rooftops. It's literally unplayable. What can I do to fix this?

bump