Azyrod_

Can confirm in 1.21.8 :

Attached testing world : just stand on pressure plate to reset scoreboards and let it run for a while to see the fastest each mob can despawn. You will see that witches can despawn in as little as 200 ticks, while zombies cannot despawn in less than 600 ticks.

Expected despawn timer for naturally spawned Witches would be the same as any other naturally spawned mob.

Can confirm in 1.21.4.

Was doing the tinted glass dark box test, while standing 35 blocks away from said box, spawned a new witch and started a stopwatch on my phone at the same time. After 10 failed attempts (due to the randomness of the despawn mecanic), I got a Witch to despawn at 14s, which is less than the 30s expected for all hostile mobs in light level of 0.

To reproduce the issue, make a pitch black room (lights level of 0). Use tinted glass so you can see through.

Disable natural mob spawning so natural spawns don't interfere with the experiment. /gamerule doMobSpawning false

Place the player at least 32 blocks from the dark room, but less than 128 blocks.

Spawn a Zombie inside the room and wait for it to despawn. The zombie will despawn after 30s mininmum. (/summon zombie ~ ~ ~32)

Try the same experiment but with a Witch. Or a Pillager. They will despawn after 10s.

I don't believe this is a duplicate of an order-dependent issue. While there is location issues with hoppers, this is a hopper Ignoring a cooldown it was subjected to.
The hopper is supposed to be in cooldown after being unpowered, so no matter the order of execution, it should wait for the next redstone tick before activating.

I agree my exemple might suffer from locality and order dependent since both the hopper and the dropper would activate in the same tick.  The issue here is that the setup is not even execution order dependent because the hopper ignores the cooldown.

Furthermore, the following setup should not have the issue of the hopper and dropper triggering in the same tick because of the added repeater. The delay should be : 1 tick from redstone torch + 1 tick from repeater + 1 tick from hopper cooldown after unlocking > 2 tick for dropper. That is not the case and using /tick freeze commands you can see the droper fires at the same time as the hopper fills it up.

This is a problem because it prevents the creation of item filters that sucks items through blocks.
If you want a hidden item filter under the floor, you have to use a minecart hopper, but you can't have a hopper below pulling items AND mesuring the minecart with a comparator for the filter setup.

I believe we should be able to do that kind of setup.