Its Definitely a Air issue, The dual stage bottle or any telescoping jack can suffer from similar issues. I would first collapse the rams completely, check the oil level, add if necessary. If you don’t follow this step, or maintain a proper oil levels you will continue to push air/oil into the valves/cylinders. Once the oil level is confirmed, with the release in the lower position pump the handle 10-12 times, this will purge the valve circuit, Check the oil level again. If full, close the release and operate the jack to full extension, rinse and repeat as necessary with the fill plug off till you don’t hear the direct exhaust of air/mixed with oil (gurgling). If you continue I’d to have problems, it could be weak pump seals Ingesting air and transferring it into the system, as long as the jacks pump seals are good and the reservoir isnt overfilled the positive/negative pressure of the reservoir won’t negatively affect the jacks overall performance. If you continue to transfer air into the system via a weak pump piston, a sealed system will eventually have performance issues.
Been there and clean that up and off of me.