Using FaceTime with Assistive Access Mode - Black Screen

I setup my grandmothers phone in assistive access and when using FaceTime the screen goes black after a couple of seconds but the call remains connected where you can still hear each other. I pin pointed that this only happens if she receives a facetime while her phone is locked. If she starts a FaceTime from her phone unlocked then everything works perfect. I called Apple customer support and they had me force restart the phone multiple times and also via iTunes with absolutely no luck. They even had me reproduce the issue and video record it to send over to the engineers but I was basically told that they see no issues and they couldn’t reproduce it. This is disappointing since I specifically upgraded her from my flip phone to an iPhone so we can enjoy FaceTime. I do see that others are having the same exact issue on the forum.