In my case we did not actually do a warm spare just Data Center redundancy. I think our issue may finally have been resolved, we had a firmware upgrade that did not actually complete on one of the MX 250s even though dashboard said it did. We did a factory default and let it do its thing from scratch and the issue resolved itself. We may have also had an issue with DHCP, I thought the MX already had a static IP address. For your case, I would make your backup the mater, and then factory reset the other one, and see if it resolves the issue. And definitely make sure you have a support case open.
