That must be a crazy amount of crystal and roast in that grist to predict a mash pH of 4.8. I assume this must use a very high amount of RO water. Are you sure that all the grain information is entered correctly and the grain type properly set? 4.8 is lower than I've ever seen predicted.
Unfortunately, I know that Ft. Lauderdale water has modest hardness and alkalinity since they lime soften their water. I provided engineering services to several of those plants in years past. So even with a higher percentage of tap water, the alkalinity might not rise high enough to counter the pH drop. Adding pickling lime might be the best alternative.