I know that you checked already but it really sounds like something is cutting into the tire. Did you feel up under the fender to see how far the seat screw is extending down. It does not have to be touching the tire when you feel for it but every time that you hit a bump the tire may momentarily scrape on that screw and over time leave a groove like you described.

 

I agree, an underinflated tire would not be worn out in the middle.

 

I compared my tire gauge with another one I had and the one I used to measure psi in my bike tire was about 4 psi too high. The Metzeler web site says I should have been running with 40 psi in the tire whereas I was running only about 34 psi. It doesn't make sense to me how an underinflated tire can cause a groove to form on the centerline of the tire.

I double-checked the clearance with the fender (I was sitting on the bike while someone else was looking under the fender) and there was plenty of clearance and no signs on the fender/screws/license bracket of any rubbing.

I just ordered another ME880 and will keep a good eye on tire pressure this time. If I can't get more than 6000 miles out of it, I'll switch to something else.

p.s. I bought my first Harley last year and they sure are a lot more expensive to maintain than I expected...

 

don't think I would have even given them a second try.
just put on the 3rd HD lop 401 on rear and 1st replacement of the front with HD lops 402 at a little over 20K, rear tires got 10k & 10.5k, all of them could have went more but I don't like to.
Got this set at Dennis Kirk and had a local INDY do install, saved about $180 compared to what the my HD dealer wanted.

 

That's why I switched from the dunlop 401's to the 880's(880's showing cord at 7000 miles) hoping to get more tire life .In 1.5 years I have about $1000.00 just in tires and that does not include mounting .But than again I was only getting about 3000 out of a set on my sport bikes.