I’m glad you got it to work on your end, but it works just fine for me without any workarounds, and it works even if I have multiple attachments in both fields.
You may want to review your automation’s run log to see what the error message says for when it fails on your end. For example, if you’re trying to update an attachment field with a duplicate attachment, it won’t let you do that.
You can see the screenshot of my automation below. It always works under all circumstances.
