I had the same issue with really bad numbers AFTER changing everything: the sediment filter, carbon filter, DI canister and membrane. I flushed them all properly during the change-out. It just didn't make sense.
I removed each filter to inspect and re-seated each properly. The two filters looked fine. However upon removing my DI cartridge from the canister I found it had NOT seated properly when I first installed it, and I had severely damaged the top seal/washer on the DI cartridge. I'm sure water was simply bypassing the DI cartridge...or at least a lot of water was, thus giving me the HI TDS numbers. I ordered a replacement pre-packed DI cartridge BRS. So...it was operator error...I created the problem by not properly seating the DI cartridge inside the canister.
My RO/DI unit "leans" downward a bit from its own weight on its metal frame, so it's difficult to properly seat the filters/cartridge without supporting the entire unit to KEEP IT LEVEL during the install. I've since devised a support to do that while I'm doing maintenance on the unit. It makes a HUGE difference...SO much easier to get the cartridges and filter properly seated!