Why does autorouter revert to 98% after reaching 100%?
Ok so I figured crazy eagle. Yes I'm using 4.16 which is old, but I found the trick.

Even though the part pad is on the correct track, there is a bug in Eagle which won't correctly autoroute the part is the track is previously made.

So the answer is to move the part so the pads do NOT touch any of the existing tracks then run the autorouter then it will complete that track.