@jokeyrhyme No problem on the barrage of questions. I took the time to investigate
A unitless zero that is not already preceded by two flex factors must be interpreted as a flex factor.
We are not treating the final zero in Philip's example correctly so we go into measuring which is where the bug for the overflow of text resides. This has already been fixed during our spec update work, but you guys can quickly address this issue on your own sites by changing the value for flex basis from a 0 to 0px and we will render the same as Chrome/FF. You can change it with your dev tools as well and see it working as expected on the live site (I commented the line that I changed in the fiddle).
The majority of the problems with flex that I have seen can be rectified by not nesting so many flex containers, or placing elements with intrinsic aspect ratio inside of an item instead of making those elements the flex-items.