Windows Server 2019 networking fails on Hyper-V 2012 (non R2) host once CU 10-2022 applied

Yes, i know... no-one should be running Server 2012 anymore.... but due to this clients ludicrous outsourcing agreement, upgrading servers is simply too expensive - so they still have some 2012 (non-R2) Hyper-V hosts.

Anyway - quite a specific issue

In short - A server 2019 guest sitting on a Hyper-V 2012 (non-R2) host will have issues with networking once Server 2019 CU 10-2022 is applied.

There is no workaround that I'm aware of - and the solution in this case was simply to move to a different host.