If the 2nd date of a "Between" pair of dates has the 'circa' checked, it can't be saved with the 'unchecked' circa. This has been tested for both the birth & death dates (presumably the burial and other dates would act the same).
The work-around is to set to the "Exact" single date, save it, then re-edit, setting the "Between" and re-enter the 2nd date (without the 'circa').
Example can be seen in the revisions of this profile: http://www.geni.com/people/Thomas-Hood/6000000012914993422