Uneven Dually tire wear

A lower psi tire make it smaller and it gets drug by the bigger tire and that dragging wears them down and overheats them. Maybe that wasn't your problem but that's why we see so many blown tire pieces parts on the roads. Did that tire ever have a leak and run for awhile at lower pressure before it was repaired? Once worn smaller, it wouldn't get any better.
You might have had a poor quality tire, also. Craig

Are the tires identical including the DOT date?

I wear outside rear tires faster too, so they get rotated amongst the rear axle.

I think it’s from driving on highways with ruts from cars with studded tires. The inner wheels ride over the ruts. Outsides take all the load.

As mentioned a over.... age on yours might a favor too.... the outside left tires might have more sun damage.... or maybe it was manufactured a little sooner than the rest meaning it’s a little older.

I would ask a tire shop but the first thing I would think of is a bent rim. Those are Alcoa's so I highly doubt that. I would make darn sure there is not something on the inner steel wheel causing the outer wheel to run out.

Don't think the OP said a darn thing about the tires being the OE skins.

On another note be darn sure to have your front aligned to 1/64" toe, NOT the 1/4" the factory specs. Factory specs cause outer edges of front tires to "STEP".

If that tire was a little smaller it will wear much faster

Ditto on..it is time for new tires. Tread isn’t the only indicator.

BTW 7 year old tires are at end of life, replace them.

A small difference in tire pressure can cause wear.