The Bug

The Spawner Minecart with an axolotl in it doesn't spawn one while underwater

Reproduce

• Go underwater
• Do

  /summon spawner_minecart ~ ~ ~ {SpawnRange:0s,MinSpawnDelay:3s,MaxSpawnDelay:3s,SpawnData:{id:"minecraft:axolotl"},SpawnCount:1s} 
  

• Wait a bit
  Notice no Axolotl spawns

Comparisons

If you want to do comparisons you can do

/summon spawner_minecart ~ ~ ~ {SpawnRange:0s,MinSpawnDelay:3s,MaxSpawnDelay:3s,SpawnData:{id:"minecraft:cod"},SpawnCount:1s} 

and notice how fast the cod spawns underwater

Note

Regular spawners spawn the Axolotl as normal it seems to just affect Spawner Minecart