Hi @swold,
My usual approach in a situation like this is to look for things that might be different for the two students... Were all of the students using identical devices with identical operating system versions and lockdown browser versions? Did all students have the same quiz availability dates/times, all of the same internet connection, etc... If you can find some differences, they could be the key. If you can't find anything different, it really is going to be hard to figure out. We've been using new quizzes for over a year, but no lockdown browser, and have not seen that issue. I'd tend to lean towards a potential Lockdown browser issue absent any other differences, but it would still be a question of why it worked for everyone else but not those two students.
I know it's not great advice, but I'd almost not investigate this too much further unless it's something that starts to repeat every day/week. There are just too many little possibilities that are almost impossible to track down to spend too much effort on a one-time issue in my opinion. If it happens frequently, there might be some things you could do to monitor for the problem and perhaps track it down at that point.
-Chris