Hot water impurities despite tankless heater?

House built in 1959, copper pipe, well water with whole house paper filter, water softener and Rinnai R94LS tankless hot water heater installed four years ago (previously had large tank heater).

Borrowed a IntelliTec digital water purity tester (tests for dissolved solids, not bacteria, microorganisms or organic chemicals). Tested last night and discovered that our cold water was at a "normal for well water" 400 ppm but that the same taps hot water read 700 ppm. Upon closer inspection, a quickly filled clear glass of hot water seems cloudy at first but then clears up within 30 seconds.

Tested again this morning from the same faucet and the cold water is now at 300 ppm and the hot at 600 ppm. FWIW, RO filtered drinking water is at 20 ppm.

Is this normal? What would cause the hot water to be worse than the cold? Scale downstream from the heater? What can be done to clean it up?

Thanks,
Concerned

Was the softener installed before, at the same time, or after the tankless water heater? Did you have the water tested before the tankless was installed? Has the tankless ever been flushed?

The softener is approx 8 yrs old vs 4 for the heater. Water tested every two years and has been fine. Have not flushed the tankless, just found the procedure in the manual.

i believe those need to be flushed yearly to stay under warranty.