Bridging issue solved, kind of a Late-Friday-Problem: Promiscuous mode was turned on, but on the wrong interface - the DMZ facing one.

It worked instantly the second I switched it off on the DMZ and instead turned it on on the target network interface.

How to turn on MAC spoofing / Promiscuous mode on Hyper-V using PowerShell

Get-VM -Name XXXXX | Get-VMNetworkAdapter | Where-Object { $_.MacAddress -eq "XXXXXXXX" } | Set-VMNetworkAdapter -MacAddressSpoofing On