In our CI we used to have some ruby commands that were piped to prepend "HH:MM:SS" to each line to track progress (because GitLab still doesn't support this out of the box, though it's supposed to land in 17.0), but it would sometimes lead to some logs being flushed with a large delay.

I knew it had something to do with buffers and it drove me nuts, but couldn't find a fix, all solutions tried didn't really work.

(Problem got solved when we got rid of ruby in CI - it was legacy).