Since the nether update i have very few mobs spawning in my farm.
All mobs?
No, i have slime, slime, slime, slime, slime, sometime skeleton, spider and creeper (2 or 3 per minute).
Before the update i had all type of mobs (spider, skeleton, zombi, creeper, witch, pillager, endermen and some slime)
Plz fix it.
Related issues
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the problem is not the design (i anticipated changes), it's mob type ratio and mob quantity.
anyway i made a slime protection and my first problem is resolved but the quantity of spawning mob is still not very good 😞.
other new problem, big slime don't take damge on magma block.
medium and small take damage on it.

It sounds like you are observing a slime chunk. You can read about them on the wiki.
Witches and endermen are rare spawns, and pillagers do not spawn underground. There was a bug preventing zombies from spawning underground in 1.16.0 that was fixed in 1.16.20.

Thank you for your report!
We're tracking this issue in MCPE-71684, so this ticket is being resolved and linked as a duplicate.
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Mob spawning and despawning has been completely revamped in 1.16. There is now a spherical (kind of donut shaped) range around the player where mobs will spawn. Outside it, they will despawn and closer to the center, they cannot spawn that close to the player. All mob farms have to be redesigned for these new mechanics.