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.