This is a major oversight in the Wall/Dimensioning ability. I'm sure everyone has made an uncased opening before, or a wall partition that is not joined on both ends. See attached screen capture of a partition next to a toilet. The only way we could work this out was to add an additional wall made of sheetrock only and aligned it to the end of the partition.
That 2'-0" dimension should be able to work in Revit without a work-a-round. Framers need dimensions to framing not finishes.