I wanted to test the new speedometer, so I made the car flip on it's roof. Got up to 350-something km/h in the RAC. I applied the brakes to stop the wheels, and downshifting there after, now I noticed that the wheels started spinning quite fast while doing so (upwards of 70 km/h).

I reset the car and made sure I was fully stopped. Shifted up to 5th gear and began to downshift to 1st, voila, got up to 25-ish km/h, did it again, now to about 35 km/h, without even touching the throttle pedal. Managed to do a lap around BL1 without stopping at anytime by only using the upshift/downshift and turning the wheel to stay on track.

I was using auto-clutch when doing this. I did try without it, but I didn't gain any speed. "Throttle blip on downshift" and "Throttle cut on upshift" were both on.

So is this a feature with auto-clutch or a bug in LFS?

Throttle blip in downshift gives you some revs when downshifting so if you do it fast enough you can keep your revs high enough to move the car.

There is another way to move your car without using the throttle pedal, you switch to manual gear shifting (not semi automatic) and hit the start engine key very fast (or hold it down), in real life you can move your car by doing something like that but your battery would die within seconds, this doesn't work on all LFS cars, in XFG it works.