I agree. I used same pH paper for both tamarix fluid and tap water in the same condition and same time, so it should be relatively acceptable result.I don't know. pH paper is only a ball park guess anyway. It is affected by moisture, sun light, and age. I have seen two different packs give different results.
My explanation is that ethylene produced from the crushed tamarix was metabolized into ethanol which change pH from acid to alkaline . I wish I have à resource to do more investigation!