I've been noticing this for quite a while now. Here is a real example from my project: https://github.com/doublep/eldev/runs/2705163604, though it's quite verbose. Also see this discussion thread with a workaround.

In short: when a program in a workflow step writes to both stdout and stderr, in the output the lines appear in mixed up order. In comparison, when the same program is executed on a local console, or e.g. on Travis CI, lines are shown up exactly in the order they are written.