Sudden tyre deflation with no puncture

Three possibilities that I see

1. You did not suddenly lose air you just did not notice until you got to the cobble stone
2. The rubber tube failed and recovered
3. The valve failed and recovered

No so sure you had a sudden loss of air. If the tire when from 90 psi to 20 psi in a short period of most likely you would have heard that.

Since the valved is the only mechanical device I going with valve failed and recovered. Either random or by force of the cobble stone the valve unseated and before it lost all air it re-seated.

I am definitely ruling out rubber tube failed and recovered.

If this has happened to you before on that tube then I would replace the valve or tube and valve. If this has happened to you on more than one tube/valve then wow - it has never happened to me.

I agree with the comment a patch could fail and then recover at a lower pressure but if you were able to air it back up and hold I would rule that out. My mountain bike was down to 20 psi once and tube only at low pressure could not find a leak but with tire on at 50 psi it did leak (at a patch).

See picture below. The little gasket may have deformed or it has a little slit that is held together with compression. Another possibility is the the valve itself (not the knob) is not screwed all the way in. Where you see the flat spot on the the threads screw that in with pliers. I had a set of tubes that by unscrewing the pump some times the valve would unscrew so I would use a dab of thread lock. I think that is actually a picture of a presta valve that the core does not remove - so screwing that in would do no good.

Same thing happened to me, but I discovered that someone had let the air out of my tire.