Due to confidentiality requirements, relatively few counties are identified in the CPS data. The codebook for each sample always lists the identifiable counties in the Appendix. Due to its small population, Ascension Parish is never included amongst these counties.
In IPUMS-USA, COUNTY must be used in conjunction with STATEFIP to uniquely identify counties. Counties with a population under 100,000 in the 2000 decennial Census will never be identified in the 2005-2011 ACS data. According to the St. Louis Fed data, Ascension Parish did not surpass 100,000 residents until 2008. Therefore, they would not have had a sufficient number of residents until the 2010 decennial Census, meaning they are not identifiable until the 2012 ACS sample.
Hope this helps.