Port scan with Networked HVAC Controllers

Hey everyone, I have an issue that none of the on-site engineers can figure out and hoping to get some new ideas to try.

We have a bunch of HVAC controllers on a separate VLAN specifically for these devices. Every day, they go offline until we run a port scan with Angry IP. Vendor has made sure these devices never go to sleep, they are always active and I have verified the settings for myself. I know they dont sleep because the scheduled AC times still run and change even when we can get to the GUI and dont ping. Once we run the port scan, pings come back and the GUI is online.

Does anyone have any ideas what may be happening here? Our network infrastructure team says its the vendors issue and the vendor says its the network teams issue and I cannot get anywhere. Thanks