Hi Jon,
The question mark notes an estimated duration -- usually because someone
didn't manually enter a duration or entered a duration with the ? following.
I agree it does seem odd that after duration is calculated or recalculated
by specifying work that the question mark should disappear. Alas, that is
not how the product works. You may suppress the display of the question
mark by going to Tools > Options, Schedule tab and uncheck "Show that tasks
have estimated durations."
I hope this helps. Let us know how you get along.
Julie