In some cases, the listed hostile mob count on the F3 debug screen remains in the dozens while more than 128 blocks above world height, even on a fresh world.

Reproduction steps (creative, minimal commands):

1. Create new world in creative mode using default worldgen
2. execute /setworldspawn
3. Make a nether portal in the spawn chunks
4. Verify hostile mobs are around the cap of 70
5. Enter the nether portal
6. Travel some distance in the nether (50 and 100 blocks have both worked for me)
7. Make a new nether portal and go through it
8. Fly up. Hostile mob count is frozen well above 0, usually in the 30s-40s, even at y=600.

Reproduction steps (creative, commands):

1. Create new world in creative mode using default worldgen
2. /setworldspawn
3. F3 + verify mob cap filled
4. /execute in minecraft:the_nether run tp @p 0 170 0
5. /execute in minecraft:overworld run tp @p 500 500 500
6. Mob count is frozen well above 0, usually in the 30s-40s

Reproduction steps (survival):

1. Start in spawn chunks
2. Have nether portal in spawn chunks, do not have mob switch
3. Observe mob cap full
4. Enter nether portal
5. Travel some distance through nether (50-100 blocks is fine)
6. make or use another nether portal to enter overworld
7. Using elytra, fly up until >128 above world height
8. Mob count is frozen well above 0, usually in the 30s-40s

Does not reproduce when leaving spawn chunks via overworld methods (flight, walking, teleports without going through the nether). Does not reproduce in 1.20. Does reproduce in 1.20.6