Resolved
For anyone still dealing with this issue...
I determined that this was just happening in Safari for me, so I disabled the last extension that I installed, which was fairly recent and since this issue started happening fairly recently, I thought there might be a connection between the two. As soon as I uninstalled that extension (RSS Menu), pinch-to-zoom started working again. I didn't even need to restart the browser. Now that isn't to say there is a problem with the Safari extension, RSS Menu, because I reinstalled it and pinch-to-zoom is still working, so try uninstalling some extensions that you've installed lately. If you haven't installed any lately, try turning them off all together, if they're already turned on.