Just because he was AWOL for the Battle of Ft. Donelson, does not mean he was on the hoof. He could have shirked the battle and remained somewhere on the premises, being surrendered with the others afterwards. If that is the case, betcha he was not very popular with his homeboys at the POW camp. The Ft. Donelson prisoners at Camp Douglas were paroled in September 1862, so it was likely the same for Johnson Island and Camp Chase.

The mystery to me is why this soldier would endure 7 months of prison and then take the Oath at the time of release when there would have been no benefit other than a Possibility of sitting out the War in shame somewhere behind Union lines, perhaps East TN.