its still something of a mystery to me as well, so don't take my word as gospel .... BUT
an RO system may not significantly alter your pH... the city tap water is buffered to be at a particular pH. (My tap is around 8.8 )
BUT the RO system WILL alter your PPM. My 500 ppm tap water goes down to about 7 ppm. Now the pH may remain almost the SAME.
BUT - it would be impossible for me to succesfully lower the pH of 500 ppm tap water. The amount of pH down needed would cause nute lockouts. (8 ml of 5% acidic vinegar would do the trick, but by the next day it would go back up to a pH of almost 8.0 again.)
WHILE - it is VERY easy for me to lower the pH of 7 ppm tap water. (A single drop of 5% acidic vinegar would almost be too much. And the pH will remain wherever it goes until otherwise altered by nutrients or such.)
[so i could see it being possible for pH to raise after being RO'd actually, by having it remove the more acidic compounds while allowing some of the more alkaline buffers to remain, though in rather small quantity]
some of this i figured out because i was buying a lot of bottled water 'to avoid pH problems' ... then i'd get it home and test it and the RO'd store bought water had a pH of 8.7 !! however, the ppm were so low (generally 15 ppm or less) that it is VERY easy to properly pH the water up or down.