From a user perspective, it can look like indeterminate or unexpected behavior - the set of environment variables available within the tmux session depends on the state of the world at the time the tmux server was started, as opposed to when the session was started. The tmux server may start when the session starts, or it may have started much earlier - as a user, unless you're aware of this behavior and paying attention, you won't know.

Other posters are right that the "correct" workflow is to set all your environment variables explicitly within the session before launching whatever process you're running, but on an ad-hoc basis, I've often found myself logged into a server and thinking "this may take a long time, I should run this in a tmux session," and this kind of behavior would have (and probably has) bitten me in that case.