using V1.4.8.2 on 1.12.2
When adding tile entity data for blocks that are replacing other blocks they just don't get saved. This means chests will generate empty, mob spawners will only spawn pigs, etc. Copy pasting the string directly from a spawner also doesn't help. I even tried directly editing the json file, but it still created a pig spawner.

I just want to create random spawners without having to make duplicates of maze segments

Can confirm this as well, even with editing the JSON.

Edit: as a temporary work around, making a single block structure with the mob spawner you want and generating it within your structure should work.