Observed behaviour:

When triggering a standard trial spawner (not an ominous trial spawner) within a naturally generated trial chamber, sometimes mobs spawn with armour. This armour can be enchanted. This was tested 3 times in one room in one trial chamber, and every time at least one mob spawned with armour (generally gold armour). This armour was never trimmed. 

When the same spawners were triggered as ominous spawners, most mobs spawned with trimmed armour. 

The world difficulty was set to normal. 

Expected behaviour:

Mobs spawned by trial spawners in a naturally generated trial chamber to only spawn with armour when they are ominous. All armour worn by mobs spawned by a trial spawner to have a chance to generate with armour trims. 

Possible explanation: 

The local difficulty for the trial chamber tested was 3.75 // 0.88 (Day 446). This could be making mobs spawned by trial spawners spawn with armour even if the spawner is not ominous.