The situation we had before was that a road built on top of an older road still had the older road under it in inactive form (no happiness produced).
that was fine for temporarily storing a more modern road (just put it on top of an older road somewhere) without needing to have a free square of city foundation. The only expense would be the happiness produced by the older road.
But that meant there had to be another rule about not placing an older road on top of a more modern one §1.
Then came the automatic sale of an older road when a newer one was placed in its location, that kinda made the §1 rule unnecessary, and apparently it got deleted (at least for some roads) try moving the older road segment you built on top of a newer road. do the never road become visible again? if it does the newer road is not lost, just hidden by an old road placed on top of it. I find that kinda likely (but I would not call it good programming).
love Villow