I'm just saying that the GH can remain the same due to uptake by plants while the pH/KH rises..................

So if was from say CaCO3, you'd keep the same Ca/Mg GH total, but the KH would go up.

Clean everything, check the rocks etc.
Tap water changes etc as well.

As I seldom test such things unless I have basic question I want answered, such issues no longer bother me. Testing can be a disease. It can cause folks to go on long excursions to nowhere.

The Flourite might have some KH causing some material in the tank to dissolve more etc due to the slight acidic nature and low KH.

You need to make sure the test kit is accurate first.

Is there some reason inside the tank that you believe has any negative issue here?

RO water membranes need cleans, and you are at about that point.
Do not assume that it's pure water...........KH is the first thing to slip through.

That seems the most likely cause.

No, plants/bacteria consume KH generally, nor produce it unless they form hard shells of CaCO3 like macro algae, snails etc and die off fast etc......

Back flush the rO and change the prefilters.

Regards,
Tom Barr