The Bug
With the introduction of new fish, dolphin, squid & glow squid spawning mechanics this has led to spawners of those entities only being able to spawn those entities at certain heights. For fishes, dolphins & squids those spawners only work between height y=50 & y=64 while glow squid spawners only work below y=30
Reproduce
Go to a body between y=50 & y=64
Do
/setblock ~ ~ ~ spawner{SpawnData:{entity:{id: "minecraft:pufferfish"}}}
✔ Notice the pufferfish spawn
Run the same command in a body of water above y=64 or below=50
❌ Notice nothing happen
Expected Result
Spawners wouldn't be height dependent when spawning mobs
Note
Tropical Fish spawners in lush caves are unaffected by this
I can confirm in 21w40a