This should be due to aging or unsuitability of the seal ring and reduced sealing ability. This can be fixed by replacing the sealing ring.
The “belly” in the subject should be the water tank of the water heater, which may also be called the liner; the "thin tube" should be the glass heat collector tube. Inconsistent concepts, especially calling things according to your own understanding, can easily affect the understanding and communication of the problem and hinder the final solution of the problem. The water tank is usually made of metal and the collection tube is made of glass. The seal between the tank and the collection tube is achieved by a rubber sealing ring. Shifting or aging of the seal ring may cause water leakage.
The principle is not complicated, but replacing the rubber ring is technical work.Very few users are able to replace it themselves, so it is not uncommon to disconnect the vacuum tube. Just ask the after-sales staff to take care of it.