I've had some users confuse the terms. We use register, log in, logout now.
Sign up and sign in confused some people because of how it mentally translated to some people's native language. We targeted more elderly people too, who didn't know the difference.
I assume that log in and log out might translate poorly to some other languages. Lots of people optimize heavily on registration, like even a 1% difference can be significant. And copy is low effort, so I assume there's a reason it evolved this way. Maybe it's a hint on which market the website targets.