i asked the same question a little while back in regards to my 9gallon cube. i have inert 1-2mm sand and some rocks yet my 7.6 ph dropped to 6.6 and it's only one third filled with plants. low light, no co2, no ferts, no nothing, if anything i underfeed my fish. you say bs but i say totally possible
Well, if the plants use all the Bicarbonate, the KH, the water will be significanntly softer, and at abmient conditions for CO2, say the CO2 is the same in both cases, and the starting KH was say 5 and now it's say 0.5, then certainly.
That's about the only real way, but rotting plant parts can acidify some waters.
But this needs to be maintained(added) fairly consistently.