An empty file is a legal file type on MacOS and iOS. The fact that there is a kind of legal file which, when present, prevents sync from working seems to me indistinguishable from a bug. Are there other instances of legal files that stop sync from working? That also seems like a security risk - if somebody can share a file the contents of which interfere with how sync works, that’s a scary attack vector.
Whether it is a code bug or a design bug is a different question.