Don't waste your money on a TDS meter.
Just buy quality DW.
Yes, DW absorbs CO2 from the air but it does that even while in a sealed plastic container. Most plastics are gas permeable. Believe it or not, so is the aluminum they make soda cans out of. Thats why they're putting best by date codes on the cans. Over time, they lose their bubbles (seriously). They tried to fix this in plastic soda bottles many years ago by making a sandwich of a couple different kinds of plastic (it worked) BUT it became a recycling nightmare.
The amount of electrolyte we use should more than compensate for whatever carbolic acid the DW has absorbed from the air.
CO2 dissolved in water is referred to as carbolic acid so you can see how it'd lower the PH of the water in the bottle.
Honestly I can't tell you whats wrong.
Did you thoroughly clean your beaker? Dishwashing liquid and scrub it a bit and rinse and then put a little 3% hydrogen peroxide in and swirl it around all over to remove any silver that may have accumulated on the glass (it does over time - not a bad idea to do this every several batches) and then a couple rinses with DW to get the tap water out (you don't want any chlorine nor minerals hanging around). It does start causing issues eventually because it works like a 3rd electrode.
When rinsing the container with DW, just put maybe an ounce in, cap it and shake it to get it to hit all the spots inside, spill out and do it a second time.