When using a Spawner or Trial Spawner to spawn Item Display, Text Display, or Block Display entities, the entities do not display inside the Spawner. 

To reproduce, use one of the below commands to place the respective block:

Block Display Spawner: /setblock ~ ~ ~ minecraft:spawner{SpawnData: {entity:{id: "block_display",block_state:{Name:dirt}}}}
Item Display Spawner: /setblock ~ ~ ~ minecraft:spawner{SpawnData: {entity:{id: "item_display",item:{id:dirt}}}}
Text Display Spawner: /setblock ~ ~ ~ minecraft:spawner{SpawnData: {entity:

{id: "text_display",text:"dirt"}

}}

Block Display Trial Spawner: /setblock ~ ~ ~ minecraft:trial_spawner{normal_config:{spawn_potentials: [{data: {entity: {id: "block_display",block_state:

{Name:dirt}

}}, weight: 1}]}}

Item Display Trial Spawner: /setblock ~ ~ ~ minecraft:trial_spawner{normal_config:{spawn_potentials: [{data: {entity: {id: "item_display",item:

{id:dirt}

}}, weight: 1}]}}{}

Text Display Trial Spawner: /setblock ~ ~ ~ minecraft:trial_spawner{normal_config:{spawn_potentials: [\{data: {entity: {id: "text_display",text:"dirt"}}, weight: 1}]}}

You will notice that the spawner appears empty, even though the entities do spawn and render in the world as expected.