right - your membrane is rejecting a very high percentagfe of the tds in the water.

Let's say your tap water read 100 ppm TDS
If your RO water read 5 ppm, then you have a 95% rejection rate.

Are you on well water?

Can you get me measure of your pH and alkalinity of your RO water? With that info we can see if you have high co2.

Does you water contain chloramines?

Russ