That size tank should deliver about 5 gallons of water as the water is let out and the pressure drops from 60 to 40 PSI. If the tank is still good, you are losing 7 GPM somewhere. Adding the extra check valve was a good way to test for water going back down the well, and apparently it isn’t. So water must be going the other way. Put a ball valve on the line to the house and close it off. Then if the pressure still bleeds off, it is going somewhere between the new check valve you added and the new ball valve.
I would think a 7 GPM leak would be easy to find, but I have seen stranger things many times. Also check that your tank is holding the 5 gallons it should. Just open a hose to a bucket as soon as the pump shuts off at 63 PSI. See how much you get before the pressure gets to 40.
You still have a leak. But if the tank is waterlogged, the leak could be very small. If the tank is still good, then it is a big leak.