Hi Joseph,
I’ve had the same issue that started a year ago. Eventually they removed the subnet, but it was blocked again a week later.
It seems to me they import a third party block list containing my subnet, which re-blocks it shortly after removal, and they won’t whitelist. They wouldn’t tell me the source (nor the reason).
I’ve repeated the remove/block cycle a few times and then just gave up and renumbered my mail servers. They don’t seem to care. At all.
Your mileage may vary though. If you do get to a solution I’d appreciate hearing about it 
Good luck!