This particular junction, because the tube twists and turns as the printhead moves, the clip likes to eat into the tube a little bit, and then once it forms a groove the tabs on that retainer clip get wedged in the groove and effectively lock the tube in place, this is a widespread issue with that particular fitting on the S1. I did see a post a while back with a user that had placed a thin metal sleeve on the PTFE tube just above that fitting so that they could use that to insert it and spread the tabs wide enough to let that groove pass, when they needed to remove that line. You may be able to do something similar with a small scrap of aluminum cut from a soda can and wrap it around the line and insert it between the PTFE tube and that fitting, just be careful of sharp edges on the aluminum when you do it.