How do you fill the water tank??
Many models have a hand operated valve that is thrown to the 'tank fill' position to fill the tank from a hose connected to the city water inlet and back to the "city water" position to use. If this is your case, that valve may be cracked open a bit or not fully closing and admitting air. The original problem of bubbles at the seam may have been due to water freezing in the pump itself or that valve over the winter if that was possible. With no obvious water leaks, you may be pumping water from the tank and back into the tank through that valve.
The pump is not shutting off simply because it is unable to build up pressure, likely due to the air (bubbles) in the system.