Proton Pass has a security flow.
If you are using a copy/past tool that remembers the last n copies (I use Jumpcut on OSX) when copying a password from Proton Pass, that password is going to be in clear in the copy/past tool.
The fact is that 1Password does not have this flow. I don't know how they do it, buy a copied password or anything else is not saved in other applications reading the clipboard. I can paste it in a webform of course (limited time to do so).