@PSX_Defector I have narrowed it down to the Persistent Reservations. This is a Nimble Storage system, and I believe that can be en/disabled per lun. AFAIK, this will be the only Windows Cluster accessing the Nimble systems in a Failover Cluster, so they probably don't have it enabled.
And yes, everything is in the same DC. Most likely in the same rack, lol... and no, not doing anything fun with iSCSI over the WAN.... although that does give me an idea, for new things to try on my own servers, lol.