By the way, if this issue only occurs when opening certain documents, it would be awesome if you could attach an example of such a document to your email as well!
Sending an email now - and no, it happens when creating a document, opening a document, loading a pixelmator file, literally anything. But it's not just Pixelmator - Final Cut Pro X is doing it too, so I wonder if there's something that changed in macOS prompting this issue..