Found this post while searching for similar issue. I have been running HA in Proxmox as a VM for at least a year now. Have quite similar setup with 3 networks. 2 of the networks are vlans for IoT and NoT devices. Reason why I am doing this is due to me using Xiaomi devices and the gateway uses mDNS/multicast to communicate, something along that line.
All was working very well until I am not sure which release of HA or could it be Unifi…the exact behavior was seen as described by @nikipore. Upon further investigation and troubleshooting, I realized that all my other systems in the network were also having almost 100% CPU usage. The minute I pulled the network cable, the CPU usage went down to <10%. This is only affecting one CPU core so you might not catch it if you do not turn on the logical processor view in task manager. Weird thing is it does not affect the system if using wifi. The moment you plug the network cable back, the CPU usage of CPU 0 will shoot up again.
I initially thought it could be the VM corrupted and I setup another new Proxmox system. All was looking good initially till I added the necessary network cards to talk to the other 2 vlans. Then I am back with the same problem. 
Could anyone else confirm if they are seeing such behaviors? Or better still how to solve it.
Edit: I was searching around and found this other thread (Multicast container causing feedback loops · Issue #1 · home-assistant/plugin-multicast · GitHub) which looks like the issue here. Will troubleshoot further. Hopefully someone also has some ideas how to solve.
Update: I was testing out a few things and I believed I have solved the issue. What I did was to turn off the mDNS option in the vlans and only have that option turned on in the main network. Now there is no more 100% CPU usage and every sensors is still working in HA. Will continue to monitor for a day and see.
