Just trying an export a large file (poster 15749x7874px cmyk 124mp) but get no preview. I did get a preview with another similar poster earlier. This APhoto file has another APhoto placed with a small graphic and a text block added. Not seen this before.
I did quit the app, then restarted but same problem. Don't have anything else open that would cause a RAM issue. The file is 20.6 MB, so not all that large.
Can upload a package to a link if desired. (BTW< it does output just fine, just no preview in export.)