Proxmox 9 Issues

I've recently upgraded proxmox from 8 to 9 and here are some of the issues I ran into

Container MTU

After the upgrade I suddenly started to experience issues with this website not loading correctly. It is running in an LXC container, and the frontend is a VM that runs nginx. After some debugging and sniffing the traffic it looked like an MTU issue, as the MSS size suddenly was 8960. In proxmox 9 the default MTU of an container is to inherit the bridge one. In proxmox 8 this was around 1500.

After forcing the MTU on the container to 1500 all problems disappeared.