This is an interesting problem. Primer-sealers can dissolve a stain if it is reactivated by the solvent in the primer, mineral spirits for oil base and water for acrylic/latex primers. It's possible this is happening.
I would try a shellac primer. The alcohol in shellac won't react with and dissolve most types of "bleeding" stains. Tinted shellac primers are available in spray cans for spot priming as well as gallons for large area.
Have the painter's use a spray can before purchasing a gallon. It could take more than one coat to fully seal a spot.
Another alternative is to use an opposite type of primer on the spots. If the painters used an oil base primer/sealer try an acrylic. Once again two or more coats may be needed. Personally, I would try the shellac first. It dries very fast and multiple coats can be applied in one day.