Hi
Not too clear about yourexplanation but I think what is happening is you aren't monitoring the output of the first vessel.
If you only check the final output from the second vessel there is a chance that the TDS in the first vessel can accumulate way beyond the tap TDS water you are putting in!!!
I have had TDS from the tap at 85 and the output from the first vessel at about 150 until I got wise.
P.