Why can UDP packets be “accumulated” somewhere, and then sent in a “heap” (Windows Hyper-V)?

Question

Hyper-V

There is a physical PC with Windows Server 2012R2 (Hyper-V and Wireshark installed).
Created a virtual PC in Hyper-V with Windows Server 2012R2 (IIS installed).
IIS hosts a backend that receives and sends UDP packets.
Through Wireshark these packets are monitored.
The problem is that after some time (~5 minutes) UDP packets stop "leaving" the virtual PC, but continue to be received by it. If you restart the virtual PC in Hyper-V, then all UDP packets that should have "left" from the virtual PC will be sent in bulk.
If you restart the back-end or IIS itself, then the UDP packets still do not "leave".