I found a similar issue MC-157265. Its status is closed. The overworld only has problems spawning when only 1 person is online in the overworld. As soon as another person enters the overworld, the issue is fixed and mobs spawn as normal.

I could not determine the solution since MC-157265 says closed to being a duplicate of other issues that are NOT the same as what we are experiencing. This world was created in 1.14.4 and never updated from a previous world. Mobs are not sticking around in unloaded chunks because it IMMEDIATELY fixes when 2 or more players are in the overworld.

For reference we are on Normal mode, mobSpawning game rule is true. Also, technically some mobs spawn on occasion. like maybe 5% of what it should be at best. The nether and the End dimension are unaffected.