Why oh why would you need a spawner to keep spawning mobs while your off line, many many times on server we have seen this, there is a reasom mods despawn when no player near & that is to save strain on server/client, mobs are entities & having lots in a chunk increases the work load for that chunk, & if for some reason some part was to fail, like turtles unload when you change dimension, or the chest/barrel/sorter to overflow, how then can the offending player fix if not even online!!, the option for spot loaders should be set on all chunk loaders in mho, while a player is online there loaded chunks work fine but when they logout all the loaded chunks should unload too, why should the online players have to deal with issues & even lag from off line players setups.
Sry for rant but its a pet hate of mine, I have been on servers where when a player logs out some one had to go check that grinders are all off, & I can tell you far more than one server, often the novice player does not even know the damage that can be caused by spillage.
So in short the the soulshard spawner as with vanilla spawners are working as intended.