The issue seems to persist with the current version (iOS, both iPad and iPhone). I use a shortcut to send an external file to a placeholder project called inbox. That works great — the attachment appears as it should in a new note.
I move the newly-created note to the appropriate project — and my attachment disappears, leaving me with the cloud icon.
Interestingly, if I drag the note back to the original location, the attachment reappears. I can then drag it (instead of moving it) to where I want it, and the attachment shows as it should.