Yes this is certainly a pitfall. You have to look carefully for that. You can spray some PB in between the tube and the fitting (guessing already been done) then while the fitting is still tight, take the tubing part and gently pry on it in a circular motion to break it free from the fitting. Do not physically bend the tubing any more than absolutely necessary.
For stuff that is rusted/stuck together in general, you typically take the approach of applying enough force to break it free. But do not underestimate the effectiveness of fatigue in these situations. Applying less force with repetition will often eventually break it free. The trick is getting enough force to fatigue the stuck joint, but not so much that you fatigue the line itself or round off the hex on the fitting.