I'm having the same issue using ionic-angular 2.2.0 and the latest WKWebview on iOS. It's working fine on desktop, android and ubuntu touch.
The row doesn't wrap when all of the columns are col-xs-12, but when they are smaller (col-sm-6+) the wrap begins to work.
EDIT: Actually, the row wraps properly when the ion-split-pane is displayed (which I have set to the default of 768px, or md). Not col-sm like I said above.