Arista 7050S Port-Channel unconfigured but blank

We have a pair (using MLAG) of older (out of support) 7050S and one of the switches is being odd with port-channels.

Sometimes when a port link goes down and then up after the port-channel has been configured for a while, the port ends up in an inactive state but with "reason unconfigured" blank.

The other side (OpenVSwitch) thinks everything is fine and tries to pass traffic over it, and fails, leading to packet loss.

It looks like this:

Port Channel Port-Channel21: Active Ports: PeerEthernet15 Configured, but inactive ports: Port Reason unconfigured ---------------- ------------------- Ethernet15 

And config:

interface Port-Channel21 description XXX-AKL-PVEC001-C switchport mode trunk switchport trunk group XXX-PVE-TRUNK port-channel lacp fallback timeout 60 mlag 21 interface Ethernet15 description IBD-AKL-PVEC001-C channel-group 21 mode active lacp rate fast 

The other switch that is part of the MLAG has the same config.

Normally I'd submit a TAC case, but given these switches are now out of support... has anyone come across this before, or can offer any suggestions?