I noticed the Nexus said PVST against the port.
So it was setting the boundary flag in the BPDU.
I converted the EX switch from default STP to RSTP.
Fix on the Cisco was to do:
Reinitializing the Protocol for MST
An MST bridge can detect that a port is at the boundary of a region when it receives a legacy BPDU or an MST BPDU that is associated with a different region. However, the STP protocol migration cannot determine whether the legacy device, which is a device that runs only IEEE 802.1D, has been removed from the link unless the legacy device is the designated switch. Enter this command to reinitialize the protocol negotiation (force the renegotiation with neighboring devices) on the entire device or on specified interfaces.
1. clear spanning-tree detected-protocol [interface interface [interface-num | port-channel]]