Dirty little secret: bike tubes are terrible. Many of them have slow leaks right out of the package. A small enough leak, in an unlucky place can be intermittent also. I suppose because of the tube flexing ever so much inside the tire from time to time. This is magnified by the fact that there is so little air volume in a bike tire, so a little lost air makes a large pressure difference.

A stuck valve is possible too.

jim