I don't know which version does this bug begin to appear, but it won't happen in 19w13b indeed
You can use this command to place a guardian spawner in the air

/setblock ~ ~ ~ minecraft:spawner{SpawnData:{id:"guardian"}}

You can see the spawner doesn't work, no guardian spawn. If you place a guardian spawner in the water, it can spawn one guardian(Sometimes it can spawn two), but only one, even if the waters is vast.
In the past, the spawner can spawn three or four guardians(unless the space is very narrow), whatever in water or air