Vertical iPad Pro shows the tablet breakpoint: all ok
Landscape iPad Pro shows the Desktop breakpoint, which is intended (all landscape tablets do).
But yes there's a Flexbox bug, I see it on mine. It's Safari that bugs with Flexbox often. So, it happens on Safari desktop too:
You can try to fix it by giving the columns a value a bit under 1/3 (3.32)
You can also try this: delete your element, and redo it from scratch with new class names, but under Safari, not Chrome. Webflow designer in Safari. This is one way to make some things work in Safari with Flex...
edit: actually that setting seems to fix it for Safari
BTW I found that you can log into Webflow on iPad pro with this browser: https://itunes.apple.com/us/app/icab-mobile-web-browser/id308111628?mt=8 Set the appropriate user agent. Don't expect much, it's for the fun :)... Editor works quite well though.