This may be true in your tank. Not mine. My TDS lowers over time as the buffering capacity is depleted and I think this is more often the case.
The KH(buffering capacity) gets depleted when there's a lot of acidification going on. The major acidifying processes in a tank are ammonia to nitrate conversion and decomposition of organics directly related bioload, feeding schedule, organics, etc. ..This can not only have an impact on Kh, but oxygen, which is even more dangerous. In your scenario, if you Kh is going down faster than you can replenish via water changes or whatever other means you've got to do it, then you may need to evaluate your tank's conditions.
A properly stocked and maintained tank with a Kh of as low as 2 should be fine. I don't deal with soft water myself, but I've happened to measure the Kh often out of curiousity and it's a constant 8 dKh. It does not get depleted after a week to let's say 6 or 5 for example or lower. It doesn't bulge at all after years, providing I do my water changes and avoid overstocking. So I don't see a reason why would yours do so, so fast? That's an inidication on its own that there's something wrong going on for the water to lose its buffering capacity so quickly.
And I know it can happen as it did in one of my tanks on which I didn't do a water change for about 2-3 months, I measured the stats out at some stage out of curiousity and the Kh had gone down to 4 after such period. However, the GH had increased at the same time by about 2dGh. And the TDS had risen as well regardless of the KH drop. The rise in TDS could have been caused not only because of the rise of GH but all the other dissolved solids that enter the water via food indirectly, processed via decomposition and then other chemical processes and stay in the water unless you take them out via a water change. If I had a way of measuring organics, it would have been quite high I bet you, despite that nitrate levels were unmeasurable because the tank is planted. If I just measure nitrates, or ammonia or nitrites and used your logic, I should have said my water quality is perfect. These were all 0. But it wasn't, because had a Kh going down to nothing, GH and TDS rising and tank loaded with organic matter, no good for the fish at all I can guarantee you that.
So because a TDS meter takes two seconds to measure and I can do it as often as I like without forgetting or being lazy, or running out of money for a liquid test. I know what happens in my tanks when the TDS changes.
If in yours the TDS goes down, then that's still an indication one can use although I can't figure out theoretically how that's possible unless your meter isn't calibrated and gives you false results.
