After fighting a wave of mobs from a trial spawner placed by a player, the trial spawner appears to lose its mob data and not function after its cooldown ends.

Step to Reproduce:
1. Place a trial spawner in creative mode and use a spawn egg (any) to tag a mob to it
2. Fight a battle with the spawner
3. Wait until cooldown ends for the spawner to be active again

Expected Result:
The spawner will return to being a waiting or active spawner with the mob data still attached

Observed Result:
The spawner appears to be empty and has no mob data yet still appears to be active