Meeting the specs and actual editing are very different. The problem I have is that Apple has made a development decision in regards how it will deal with media, for example prores 4444 with alpha files are fine when left in the native or unconverted codec, but when the timeline is set to view-proxy, the file needs to be converted into a proxy prores format, and this conversion strips the alpha channel from the prores 4444 with alpha channel.
It seems that no one has picked up on this disaster in the life of FCPX, surely if you're going to down res a codec from 4444 to proxy, you would endeavour to maintain the alpha channel in some form.
When editing above HD, that is, in proxy mode, I find it helpful to create masks when I need to use a prores 4444 with alpha file, this way I can at least edit and see what I am constructing.
For a long time I was confused as to why, and the only solution to my confusion was that Apple strips the alpha channel from the prores 4444 when converted to proxy.
Now Apple has to decide going forward one of 2 ways forward, either carry on with the proxy timeline view, and fix the prores 4444 with alpha -proxy prores bug, or find a way to to not have to convert the prores 4444 with alpha into a cached file that plays on the timeline in proxy mode.
I am not for 1 second saying the proxy mode is a bad idea, NO NO, it is a huge great idea, flawed by bad codec design! Something I keep hoping will be fixed!!