I have a 2013 FX, a 2016 SR and recently acquired 2014 S That Hollywood Electrics just did a bunch of upgrades to.
I have never had any belt issues, the 2013 is over 10,000 miles, I believe the SR is similar, probably less. The 2014 has
a little over 9,000 and as near as I can tell, original belt. I have always used the tensioner originally sold and recommended
by Zero. I tried the app but it appeared ( to me ) to be less accurate.
As far as center tracking, when you apply power, torque is applied trying to move the wheel to the right. This will try to move
the belt to the outside of the sprocket. If you were to center it under power, then it would tend to move inward if ridden gently.
If centered for gentle riding, then it would tend to want to go outward under power. This would probably explain why it doesn't
appear to stay centered. In the 70s and 80s the chassis were so bad that I would have to start at one side of the drag strip
with the bike pointed just a bit to that side to compensate for the chassis flexing under power.
By the end of the run, I was at the opposite side of the lane.
What I do with mine is to center the belt, then go do a few launches to see how it ends up, then adjust again if necessary
Another issue can be that after you adjust it, when you tighten the axel, it may tend to move just a touch