I created a custom spawner using a command generator. It worked and I had a custom endermite spawner. They were had a custom name, custom name color and custom drop. The spawner was configured too, I changed it's spawnRadius, maxnearbyentities and such. When I set it to night time, the first ones spawned and the spawner immediately turned into a pig spawner and started spawning pigs.
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Duplicates of MC-92282. You should put the command you are using, but you mostly likely did not specify a correct SpawnPotentials tag.