Something like this can depend on which format and other decisions are made during encoding. And be a bugger to puzzle out, at times. Which gets very annoying quickly.
I know some users have found that ProRes in mxf does a lot better for alpha and for moving between video post apps.
Hmmm... nothing has changed about file format support for Apple ProRes.
It's been awhile since I've exported ProRes 4444 from Motion, but I'll give it try and report back.
Back before Apple deprecated QuickTime, we could simply change the .motn extension to .mov and the file imported into After Effects or Premiere Pro. It's really unforunate that we lost that functionality.
Thanks Warren! I also tested your file and it does work, but for some reason mine doesn't. I did the same operation and same format in AE and it now works, but not in with particualr motion file.