Ring 3 Plus issue with "This footage is unavailable"

I just installed a Ring 3 plus doorbell for my parents recently. It is connected to the hard wired lines of their previous doorbell....Everything seems to work fine until my 2 recent visits to their houses. I noticed both of those 2 visits, there was no video captured. I saw a big gap of "This footage is unavailable" error message in the app. This is unbelievable and I don't know why. I own 2 Ring Pro devices while my dad also has a Ring floodlight, but we never saw this issue before....What's going on? I ran all my troubleshootings. The power was fine. The wifi signal was excellent....could this be a defective device or what?