If I understand correctly, with the tender on you measure 13.5 volts. If you pull it off you measure 12.0 volts soon after with batt terminals removed? I assume your meter is calibrated or close and reads 12.5+ on other battery's.

If that is it, sounds like it could be several things (easy things first) 1) new battery is not yet fully charged. Leave it on the tender and try again or better yet use a known good conventional slow rate battery charger and measure again. 2) Battery terminals have high resistance... poor connection or dirty 3) Battery is bad.