used twin di for years & that has never happened, are they before the tank or after the pump ?
Check to see if the pipe inside the vessel is fitting correctly not leaking , if it comes loose the water fills up in the vessel and any newwater coming in just goes straight through without being filtered.
Are you using Tulsion resin by any chance?
Do any of the Two vessels have Tulsion in?
Tulsion has dropped through the floor quality wise. Myself and many others have experienced what you now have. I don't buy it anymore. I would see how you get on with the unger, might be an idea to test the TDS of the vessel which had Tulsion in it.