The bug

I placed a drowned spawner with drowned spawn egg and command,then I changed the data of drowned made the spawner spawn a drowned with a trident by using "data" command. But when I far away from the spawner and return, the data of spawner got lost and it spawn normal drowned.

How to reproduce

1. Place a spawner

   /setblock ~ ~ ~ spawner
   

2. Right click this spawner with a drowned spawn egg
3. Use this command to change the spawned drowned by this spawner

   /data merge block x y z {SpawnData:{HandItems:[{id:"trident",Count:1},{}]}}
   

   → It will spawn drowned holding a trident

4. Move far away from this spawner
5. Go back to the spawner
   → It will spawn empty-handed drowned, not drowned holding a trident

Note: it won't happene if I use this command directly:

/setblock ~ ~ ~ spawner{SpawnData:{id:"drowned",HandItems:[{id:"trident",Count:1},{}]}}