Montgomery County, Mississippi was created 13 May 1871 from Carroll and Choctaw counties. Poplar Creek was in Choctaw County from 1833 to 13 May 1871 when Montgomery was created. Poplar Creek didn't move, but it was then in Montgomery County.
You would expect a 1868 marriage record to be in Choctaw County, since Montgomery didn't exist.
I have not used Choctaw County records, so I don't know if they are complete or not.