This a bug that has been bothering me for some time now. I recenlty found it posted under the Java Edition Bugs under ticket MC-273635:

How to reproduce:

1. Create a new world in creative mode.
2. Get a trial spawner and a spawn egg of your choice.
3. Place down the trial spawner, right click the block with the spawn egg.
4. Change into survival mode.
5. Kill the mobs spawned by trial spawner.
6. Wait until the recharge time is over.
7. Execute /data get block <coordinates> <of> <trial spawner>.

Observed result:
You will notice that the spawner will turn its block state into "waiting_for_player" for a quick moment before going on inactive again.
You will see that the trial spawner's entry for mob to spawn is empty.
The same applies when the block is placed with /setblock or /fill.

Expected result:
The trial spawner does not forget its designated mob