Yes! You've pointed out exactly one of the best examples. This is why I think some sequel attemps have diverged from Stunts so much. Stunts's physics engine is, strictly speaking, full of bugs, or more honestly, made out of bugs. And yet, the result is amazing. The developers of Stunts just tried to patch things quickly... They didn't expect these things to become the personality in Stunts, but actually wanted to make them less noticeable. After all these years, we've come to love these things and now, if I'm to develop an engine, I'll try to reproduce this... but I wouldn't do it through quick patching, as that would result surely in a new, totally different set of bugs, much unlike those of Stunts. What I would do is start by trying to create something more faithful to real physics and then, make the necessary touches to it so that it results in something that feels like Stunts.
I've tried stopping completely while upside down in loops and pipes. When you reach a certain low speed, the car detaches front-first from the ceiling and falls to the ground. But it is very clear that this "gravity" does not become active until you lower your speed considerably. To see this, drive into a long pipe and as soon as you enter, roll until you're upside down on the ceiling. Then remain on the ceiling for the rest of the length of the pipe. You'll find that, no matter how long, as long as you maintain a reasonable speed (not too high), the car will stay on the ceiling.