I ran into this in my lab when upgrading a switch which caused a fabric-segmentation and obviously the release notes of a previous version show:
Other Important Notes and Recommendations
Management Server Platform Capability support changes in FOS v6.4
FOS v6.4 no longer automatically enables the Management Server (MS) Platform capability when a switch attempts to join a fabric that has these services enabled. This prevents a FOS v6.4 switch from joining such a fabric, and ISL will be disabled with a RAS log message. To allow a FOS v6.4 switch to join such fabrics msPlMgmtActivate command should be used to enable the Management Server platform services explicitly.