No one seems to have a definitive answer, so I'll add my .02.
I've foliar fed plants in the middle of summer, 90F+ weather, and they don't burn like that.
I think people only get actual pH issues when they really fuck up and way overfeed a plant. A lot of people will blame pH when they don't understand what's going on. But unless you've been pissing in that soil or dumping shit in it before using it to pot your plants, i dont know that pH is the culprit. Where'd you get the soil? Like the guy above said, flushing cant hurt, but it's not going to remove anything unless there was something wrong there in the first place
I've got some sativas (Mau-mau from Blazing Pistileros) that get really finicky for calcium. They kind of start looking like that even as seedlings (as soon as 5 nodes), if I dont supplement their water with calcium nitrate.
If your tap water really that bad? You might want to just stop using R/O. Most people don't really need it, and guys at the hydro store just want you to buy stuff. All it's doing is filtering our your calcium so you have to add it back.