I wonder if the Antenna Analyser is being fooled by either common mode signals on the coax or actual RF being fed into it. I had a lot of problems tuning a big 70 cm antenna when all the analysers showed rubbish results - RF pickup. Testing it using my Bird Thruline and it tuned up perfectly.
Get well away from the antenna (but then coax loss might fool you unless it is really low loss) or, my preferred option, get some sort of choke balun close to the antenna to get rid of common mode signals but, whatever you do, make sure you and the test gear are well away from the radiated field from the Yagi to keep radiated RF out of the test gear.