I was actually trying to join 2 clips - the grade applied to the correct clip was just a fortunate byproduct.
I had split up a takes from a 5 min clip. But found after grading I had split a clip in the wrong spot and was trying to join it back up.
I'm not sure if I should always place the playhead at the end of the clip nearest to the one I want to join; always use the second clip; select one clip, then place the playhead over the other clip and click "join clip", etc. It also seems there is a way to control which grade is applied to which clip.