Hi Chris,
The multi select list box is a repeating field, so when you promote the property (so that it becomes a column in SharePoint), you are asked how to handle this. See the attached screenshot. You can select: first, count, merge or last.
Which one have you selected and is it not providing the expected behavior?
first displays the first item in the list
count displays the count of items selected
merge will merge all the data in the list
last will select the last item
Hope this helps!