Absolutely it's ok
We bottle our Kvass flat, cause we know that once bottled & refrigerated the Kvass will continue to ferment slowly.
This basically means the beneficial microbes whilst metabolizing more carbohydrates produce more lactic acid AND CO2. It is the CO2 which provides the FIZZ.
In the warmer weather you may find that your KVASS develops FIZZ a little faster in the fridge than during Winter.
WHY?: MICROBES are more active in warmer conditions.

Some folks prefer their KVASS fizzy whilst others prefer it flatter (without the fizz)
IMPORTANT THING TO KNOW is that BOTH ARE FINE to drink.

No. 1 rule whenever opening a lid of any fermented drink - OPEN LID SLOWLY. Especially deep purple BEET KVASS😱
DO NOT SHAKE THE BOTTLE BEFORE OPENING!!!

If you want to incorporate the sediment settled on the bottom of your bottle - just tip the bottle slowly to release sediment.

BEST to keep your bottles standing up in the fridge rather than laying down - just in case they leak!! (eeeek)