A broken screen or failed seal in the upper distributor can allow water softener resin beads to escape into the water supply. A cracked upper distributor also can cause this problem.
Unplug the water softener and shut off the water supply, then remove the valve assembly and inspect the upper distributor components. Replace the screen if it's torn or damaged. Replace the screen seal if it's damaged. Replace the upper distributor if it's cracked.
You'll need to add resin beads to the filter or replace the resin beads after repairing the upper distributor.