I actually don't currently have a 300blk upper. I have built quite a few for friends and family. When I want to shoot one, I borrow the 12.5" upper I built for my father. It was built on a Bergara barrel with pistol length gas. And it runs in all four possible modes: subs / supers / suppressed / unsuppressed. I've never had a factory sub load go super with or without the suppressor mounted.
I can see how it might happen with handloads. If you handloaded subs that were just below the super threshold, any change in MV could push one fast enough to crack. Whether it was because you dropped a couple more grains of powder or due to freebore boost of a suppressor. But that's a load that would already be pushing the envelope of velocity. Most factory subs are closer to 1000 fps, with enough cushion to stay subsonic even when suppressed. I mean, that is the point of 300blk, isn't it?