At least one other #pCloud user in r/pcloud reports seeing foreign files on pCloud for macOS (unsure what version).
I have screenshots of someone else's data files appearing on my computer. Many other people have posted screenshots. This is not a hoax.
I'm not going to open a Reddit account and wade into the cesspool that is Reddit, not even for this. It's enough for me to see multiple confirmations of the problem I personally experienced to know that it's real and widespread.