Good call to have confidence in your growing conditions before flowering.
I've seen people have problems with grocery store RO water. Let's just say I don't trust the store to service the machine properly. If it's used very heavily, the change needs to happen sooner than on a unit that gets only light use. If they aren't testing the pH and EC daily, they really have no business selling their so-called purified water, lol. Water that has been truly deionized and then allowed to air equilibrate will actually register a slightly acidic pH due to the presence of carbonic acid from atmospheric CO2.
Just occurred to me... is your tap water like really REALLY bad that you are running RO in a soil grow? That's not what people usually do. Tap water has minerals that the plants do need, and one of them is dissolved CaCO3 (limestone) that actually helps buffer the soil from becoming too acidic, and the plants REALLY need the calcium. If you are going to continue running the RO, you need to add calmag plus.
Think about switching to tap water, and on the next transplant, to a more stable soil mix.