1 trailer brake locks up

If that's the only wheel with new shoes than they can't all be adjusted the same since the wear will be minimal vs the others. The stroke will be very different compared to the older worn shoes. Adjust them yourself. Turn the adjustment nut until the shoe touches the drum then back off 1/2 turn. Do the same for all four wheel ends.

 

Could be faulty brake valve on trailer had to disconnect the blue line to get brakes to release on forward axle of set of tandems.

 

"brakes worked fine before work got done." So highly unlikely that it is anything else than what was worked on, meaning not an air problem or anything else.
There are different friction ratings on brake lining and pads. Possible they used one that is more aggressive than is on the other wheels. Or as you mention a bad/damaged brake shoe. Before pulling the wheel I would 1) grease the slack adjusters and s cams 2) Set the tractor brakes but not the trailer brakes. Have another person apply the trolley valve or foot brake and compare the wheel that locks up to the others. Is it applying at the same rate and releasing like the others? If someone pulls the wheel they should closely check the cam bushings, splines, rollers (maybe wrong ones, actually any part they replaced when doing seal) and drum. If the friction rating of the other shoes on the other 3 wheels is not visible it may end up being better to replace the ones on both sides of that axle.

 

Take it back to the original shop and force them to make it right or refund you. Why are you driving around with bad brakes?

 

Do the new brakes stay locked up, or do they release with the other 3 brakes as they should?

New brakes with old brake drum or new brake drum….?

 

@chowblower Is this trailer air suspension? If so, have you made sure the air bag at that position is inflated?

How heavy are you loaded normally?

 

I agree. Change occurred at the point work/replacements.
IMO, before the new corner gets any more wear, replace the other 3 corners with the same components. Same brads, part number etc.
Only other issue could be antilock sensors.