I had a similar experience just 2 weeks ago with an LTS 32 Channel (HIK OEM) NVR. It had been installed for over a year with no internet, so can't swear it could have existed and didn't know about it. I added some cameras and did a firmware upgrade and connected it to the customers new internet router with port forwarding and LTS DDNS. Only 2 of the cameras were direct connected to NVR and the others were in a separate POE Switch.
Everything went fine. Set it up on IVMS 4500 and the customers began logging in. Then I got a call that it was only showing 1 camera. I later logged in from my phone and it was fine. Then it went back to 1 phone. What's crazier, is that when I logged in on a browser with the DDNS, it displayed the one camera log on screen instead of the DVR. It's like it port forwarded randomly through to this one camera.
I had virtual server turned on and that may have somehow been doing it. I turned it off, rearranged some cameras and the problem seems to have gone away.
I would just like to know what was going on there.