On top of the DI resin chamber there should be a gasket like this.
It could be damaged or a crack in that lid.
Water should flow from the bottom and out through the top, where the gasket is.
This is a picture of the bottom where water enters.
The only way to consume the top DI first is by it being hooked up reverse, which you ruled out. Or the gasket is missing or the lid is busted. Allowing water to bypass the DI and it only pulling some of the TDS out. Which is why you still have TDS after the resin.
It is possible the top you checked "in and out" is flawed on the inside but not very likely.