Nordic brown bees are rather rare and your neighboring beekeepers are likely keeping other subspecies or lines of bees, like Italian, Carnica, or Buckfast bees. This increases the risk of uncontrolled cross-breeding occurring, which should be avoided, since crossbred bees sometimes exhibit aggressive behaviour and ultimately are harder to manage. Also if we are interested in conserving the Nordic Brown bee with all of its adaptations it is important to not end up with hybrids.
A colony with a purebred and puremated queen will always produce purebred offspring.
A purebred queen that freely mated in a drone aggregation area could possibly mate with non-purebred drones. Her female offspring would then be cross-bred/hybrids, but her drone offspring would still be purebred. This is due to the fact that drones are a genetic copy of their mother and they receive no genes from their father at all; while the female offspring receive genes from both the mother and father. The signs of cross-breeding show in the worker bees (always female) who have both the maternal genes from the queen and the paternal genes from drones that represent some other subspecies.