I found the bug in 1.12.2 version of modded server when playing with friends
Verified that the bug still persists in current vanilla 1.16.4 version.

When player is in survival mode without cheats turned on or being an op in the world, mob spawn eggs don't work properly on spawners.
Spawners will change to desired mob type for one spawn cycle only before changing back to original spawner mob.
If player has cheats/op/creative, the spawner stays as the desired/egg mob after spawning first cycle.

Spawn egg will be consumed in either case