Proxmox Virtual Environment 6.1-3 / Debian 10 (buster)
Kernel 5.3.10-1-pve
Ethernet controller: Intel Corporation Ethernet Connection (2) I218-V (rev 05)
Ethernet driver: e1000e 3.2.6-k
Ethernet firmware: 0.1-4
The solution I've found was to create a oneshot service which disables segmentation offloading.