"Used up resin adds tds to the water."
Corrictimundo.
Seen it loads of times.
Once the resin is spent it leaches back all the ions it has taken out of the water, the ions in the input water have a stronger charge than the ions now trapped in the resin, the resin can no longer hold them and the charge pulls them back out.
My water is about 100 out of the tap but I have seen it come out at 270 from the DI vessel when the resin is spent, and it happens quick often in a day or two.