Live Migrate fails with event 21502 (2019-->2016 host)

I have 2016 Functional level cluster with Server 2019 (basically in a process of replacing 2016 host with 2019)

If VM is running on 2019 host I can poweroff, quick migrate to 2016 host, power on & all is good

But Live migration always gives me above error

All I am getting in Event Data is (very descriptive?!):

Live migration of 'Virtual Machine Test' failed.

Nothing else, no reason.

If VM is running on 2016 host I CAN do live migration to 2019 fine! (albeit with errors reported in this thread, but I do NOT have VMM being used!)

vm\service\ethernet\vmethernetswitchutilities.cpp(124)\vmms.exe!00007FF7EA3C2030: (caller: 00007FF7EA40EC65) ReturnHr(138) tid(2980) 80070002 The system cannot find the file specified.
    Msg:[vm\service\ethernet\vmethernetswitchutilities.cpp(78)\vmms.exe!00007FF7EA423BE0: (caller: 00007FF7EA328FEE) Exception(7525) tid(2980) 80070002 The system cannot find the file specified.
] 

Both host are IDENTICAL hardware on same firmware level of every component!

There is NOTHING relating to even attempting migration in local host Hyper-V VMMS/Admin/Operational logs

In Hyper-V High Availability/Admin I get same error but with Even ID 21111

Seb