I was playing on a 1.16.3 server with some friends and we all experienced some bugs concering mobs spawnings, that is:

• Creepers and Skeletons spawning too close to players (1-2 block distance, or even directly on the same block)
• One single magma cube spawning in the overworld
• An unusual amount of mobs spawning inside walls (skeletons, creepers, zombies and endermen), both inside caves and outdoors
• One evoker spawned outside, far from any village and mansion and not during any raid.

Related to the magma cube: we didn't build any nether portal yet in this world and it spawned close to y=10 depth (while digging straight 1block-wide/2blocks-tall corridors looking for minerals), Creepers and Skeletons spawned too close to players both in said corridors and outdoors.

We also expercienced mobs spawning in well-lit areas (such as the magma cube in the aforementioned corridor)