Mobs will not spawn from spawners if the spawners custom spawn rule light value is less than 15 and they are under open sky.  So the block level is 0 and the client screen display is pitch black with stars, the "skylight" value is 15 and blocks spawning.

   
Then when you put a roof over it, blocking the "skylight", it will begin spawning.

/setblock ~1 ~1 ~ spawner{SpawnData:{entity:

{id:"minecraft:skeleton"}

,custom_spawn_rules:

{block_light_limit:[0,14],sky_light_limit:[0,14]}

}}

 

This example without custom spawn rules will spawn mobs at night under an open sky.

/setblock ~1 ~1 ~ spawner{SpawnData:{entity:

{id:"minecraft:skeleton"}

}}

 

The bug is in the BaseSpawner class in the serverTick method.

 

It should probably use the calculated skylight value adjusted for time of day and weather like normal spawning.

.