I see this same behavior. I have 4 built in NICs and 2 10Gb via add-in card. I use 1 of the 1Gbps built in NICs for a direct connect to a backup NAS and the RTSP stream tries to use this instead of the correct 10Gbps NIC thats connected to the main network. Disabling the 1Gbps NIC let’s it switch to the right NIC but I need that one back on and no setting changes in DSM seem to make a difference.
For some reason the HA integration is not using the configured IP address and is instead gathering the IP from DSM somehow and choosing the wrong one (first found?) when multiple are present.