Issues with "Hyper-V" networking only very selectively passing traffic.

Good day.

I'm working on a PoC Citrix installation. There's a "Windows Server 2025" VM (running on "VMware", with all CPU virtualisation features exposed to the VM), on which I've installed the "Hyper-V" role and "Network Virtualization" feature (required so it can be managed by "System Centre 2025"'s "Virtual Machine Manager"). I've created an "external" switch within "Hyper-V", which is bound to the second network card in the "Windows Server 2025" VM:

As you can see, SR-IOV is disabled at the switch level. I've created some VMs, all of which have SR-IOV disabled at the VM level; E.g.:

All other settings are left at their default values (E.g., "Enable virtual machine queue"), though I have changed the dynamic MAC address range so as not to cause any potential conflicts with any other "Hyper-V" hypervisors on the network.

So, all perfectly straightforward a setup. If I enable "Allow management operating system..." then the "Windows Server 2025" DHCPs an IP address on that interface, which I can "ping" both from the server itself, elsewhere on the network, and from a VM... well, if I manually assign an IP address to the VM anyway, because that's the issue here - traffic from any VM simply does not make it off the server. Mostly... Let me clarify:

Install a VM and it gets a NIC with an appropriate MAC address, but can't DHCP for an IP address - the traffic never leaves the server. I know it never leaves the server because I've installed "WireShark" and when I capture that NIC (assuming "Allow management operating system..." is ticked, obviously...) there's no DHCP traffic.
OK, assign a static IP address (temporarily "borrowed" from our DHCP server) to the NIC in the VM and try "ping"ing the default gateway. No corresponding traffic on the server NIC...
However, what makes this really weird is that, with a manually assigned IP address, I do see SSDP traffic from that VM going out over the server NIC:

(10.100.10.109 is the address I manually assigned to the VM).

So it's really, really weird - only SSDP traffic is making it off the VM and onto the LAN - literally everything else (including DHCP packets...) is getting lost somewhere.

I've removed and recreated the "Hyper-V" external switch; I've enabled and disabled IO-SRV; I've prodded and poked at this pretty much every way I can and I can't see any reason for this behaviour. I regularly use "Hyper-V", so I'm used to many of its little... quirks... but this isn't one I've seen before. However where I've used "Hyper-V" before it's always been on physical hardware rather than, as here, on virtual, so I can only guess that might have something to do with it, though honestly I can't imagine what...

The VM I originally created was a "Windows 11 22H2" system (installed with no networking, obviously...). With no networking, I can't "Windows Update" it, so I downloaded "Windows 11 25H2" and spun up another VM using that, just to see whether it was a "Windows 11 22H2" issue. Nope - "25H2" behaves the same way. I've also spun up a "Red Hat 10" VM on there for comparison, and that also behaves the same way - no networking.

This is the second system where I've had this issue - the Citrix PoC started as a single "Windows 2025 Server system, but I had exactly the same problem with "Hyper-V" networking on there, so I built a second "Windows 2025 Server" just to run "Hyper-V" in case it was some conflict between "Hyper-V" and the Citrix virtualisation software. Apparently not...

Any ideas, anyone? Thanks in advance!