Wow - if they can't find any issues, why can't they keep that R5 and send you a new one, or certified refurbished one (at your choice)? That'd make me a lot happier if I were in your shoes. I assume platinum CPS is a add-on service plan, right? You'd really think they could swap bodies to make sure it doesn't happen again.
Actually, if this is a bug in the firmware, as opposed to just a glitch in that particular camera, swapping bodies would accomplish nothing other than to cost Canon money.
And my suspicion is it really is a bug in the firmware, since the problem is so prevalent. It's intermittent, though, which makes it hard to reliably reproduce the problem, which is the first step to debugging it. There's probably some set of steps one can take to cause it to happen, and some of us follow that sequence (without realizing it) and some of us don't, since we all have different habits.
So (again, my suspicion) one part of Canon probably realizes there is an issue and is struggling to get a handle on it, while the repair shop runs standard tests that aren't showing anything wrong--they don't have the software people there working on the camera itself, so they're doing the only thing they can--run the standardized tests, and repair if a problem is found, send it back if not.
I know many people have had frustrations with CPS in the past, refusing to fix a sub-par lens for instance, but this is probably a very different situation.
Which does not make it any less frustrating. (And I say this as someone who did have a freezeup once, and probably would have had more if he were to use the camera more.)