I'm currently editing a project that was shot on Arri Alexa Mini. The footage spec is ProRes4444XQ, 2944x2160, 4:3 squeezed anamorphic. The actual frame size for the film I'm editing is 2797x1052 in 2.40:1 ratio. Because the original footage was squeezed in order to pack more pixels, I had to un-squeeze the footage by selecting "Widescreen" in "Anamorphic Override" setting in video info tab which gave me a un-squeezed 2.40:1 anamorphic frame for editing.
However, the problem I encountered was when I generated proxies for my client. All of a sudden, my dailies became just black screens (turned green when skimmed through). I contacted Apple for tech support and had a close to 2 hrs of trouble shooting with FCPX senior advisor and we found out that when "Widescreen" is selected in "Anamorphic Override" setting, all your proxy files turn black.
The only way to make the proxy files visible again was to change the "Anamorphic Override" setting to "Off." Then of course, your anamorphic footage will be squeezed again so that was not an option.
To be clear, I'm only talking about "Proxies." All my original medias appear correctly, just the way I want but only if when I switch it to "Proxy." Since the project is a short film and my system can handle the raw footage so this won't be a big deal but if it was a feature film then this would have been a serious problem down the road. One of the best features about FCPX is the ability to switch back and forth between your proxy files and your original media files with a click so I really hope this bug gets fixed soon.