Sort field disappears sporadically

Occasionally, one or more of my sort fields will either disappear or be replaced by one I didn't select. This only occurs when I open, close, and reopen different schedules. Most of our schedule layouts are sorted by Start and Total Float. Usually, the Total Float field will disappear and just the Start field will be left. Sometime both are gone and are replaced by one "off the wall" field. (I attached a screen shot of the Sort dialogue box in case there's any question about what I mean by Sort.)

I would assume this is a known bug, but I couldn't find anything on it in the Knowledge base. If this is not a known issue, I'd like to have this become a Service Request or Program Enhancement Request. Which one would be appropriate for this problem?