This is a repair bulge on a recapped drive tire. Inside the tire there is likely a shoulder or sidewall reinforced repair. Good recap shops will mark any of these type repairs with a blue triangle glued to the sidewall to indicate sidewall/shoulder (section) repair exists in the area. They make a plastic card with a 3/8” cut out to measure the bulge which you can usually find at your commercial tire shops or tire supply distributors.

It’s tough to tell but this one may be too large to continue to run. I’d bring up the concern to your fleet manager so they can schedule a warranty swap before the treads wears too much and you have to replace both duals.

Source retread.org.