Is there still a residue? Try mineral spirits, acetone (some finger nail polish contains it ), isopropyl (rubbing) alcohol works surprisingly well on some things. Mineral oil, WD 40..like removes like.

Not much you can do, except refinish the stair treads. The stain was never completely dried before the polyurethane was applied on top of it. Not sure what the original working condition was. But, insufficient heat in the work area, bad weather or simply not allowing enough time for the stain to dry can be the cause. Dark stain colors take 18 or more hours under good conditions to dry properly. Any of the "good" is off, allow extra time.

Then polyurethane is now damaged. A light sand and a coat of polyurethane would make them look 90% better, a full refinish would make them brand new.

Sand the treads and re-stain. I would polyeurothane, which might make them slippery.

I do not have a lot of ideas but maybe renew the stairs.