I looked at Elijah's Compiled Service Records; there are some records for him also in the 17th Consolidated Regiment, Texas Dismounted Cavalry. My expertise is mainly with the 24th, but the CSRs are similar.

I don't see any evidence that Elijah fought at Arkansas Post, although you may have evidence from a different source. If he had been captured, you would know it because the records of the Feds were not lost. Part of the 17th regiment was not captured at Arkansas Post. It is possible that he was on detail at the time of the battle, or that he was with the part of the regiment that was assigned elsewhere. From that point, (beginning after the exchange of prisoners in Virginia in May 1863) all his CSRs have a note that he was in the Trans-Mississippi. This means he was serving in Arkansas, Louisiana, or Texas. The final muster report shows that he was still in service in 1864. For more information on what he was doing, you will have to locate regimental histories, memoirs, testimonials, letters, pension records of other soldiers, etc. You can also read the reports filed by his captains, which are included in the CSRs.

My webpages on the history of Company B of the 24th may be of some help to you.

Good Luck!