If this is a small company, I would probably not worry too much about process and ask the concerned employee to discuss this in a friendly and open way.
Maybe a misunderstanding which can be cleared, maybe the reason is actually lying deeper. Good chance you can resolve this by being emphatic and act understandingly.
That being said, I would note down a memory protocol afterwards in case this turns ugly after all.
That being said, I would note down a memory protocol afterwards in case this turns ugly after all.