I wasn’t familiar with the phone model, so I looked up the Samsung J7V. It appears to be one of the budget models from 2017 built specifically for Verizon. My guess is that the phone is not capable of switching between 2 4K videos, which entails having both videos loaded at the same time. So going from the first to the second scene, if they were 4K videos, could be problematic. We have seen this with older or very low spec phones.
For that device, try using HD videos rather than 4K, as it won’t have the horsepower to reliably run 4K. Unfortunately, if you will have to support older model phones, you’ll need to look at content that can run on those phones.
You mentioned that your bosses phone was the newest iPhone, and that it was working properly on that device. Is that the iPhone XR you are referring to?
Also, what is the resolution of the videos, have you tried running the CVR app on the devices?