Solved! Go to Solution.
Still is an issue in version 10.5.1. Disappointing.
The Join Field tool can be VERY slow, perhaps defeating your reason for wanting to run in the background. Recommend at least using the "in_memory" workspace to help performance.
A workaround might be to just create a new feature layer AFTER (precondition) you do your calc., which I assume is your reason for a temporary join. The new feature layer will have your calced field and no lingering join.