The current implementation does not support stacked NVRs, which is a feature release by UniFi 2 years ago.
All cameras attached to the stacked NVR are currently not been seen by the integration.
I wanted the same feature, but during my attempts to get it working, I changed the protect local user password for HA and broke the configuration. After some time playing around to try to re-auth, I gave up and deleted the integration from HA and then re-added it. All the cameras from the stacked device turned up … YMMV
Can you have 2 protect OS instances running when you migrate to the stacked NVR or do you have to uninstall the protect OS from the dream machine router? I’ve got the same issue where I can’t get the cameras to pull up from the stacked NVR. It still continues to pull the previous instance.