That was something that was a puzzle to me several years ago. As I am sure you know that the Richmond Government had been in 1862 trying to get troops from everywhere they could for the A.N.V. This is one of the reasons for the change in policy to combine several Departments such as Van Dorn going to Mississippi to make larger armies to counter the Union Armies such as Grant and McClellan.
As you probably also know that Sec. of War Randolph In the O.R.'s had ordered Hindman and Holmes to provide 7 Regiments primarily from Texas for the eastern theaters. 3 of those 7 regiment were suppossed to be Garland Texas Brigade as soon as they were able to be relieved at Arkansas Post. There had been several plans of how to use these regiments from using them to advance up the east bank of the Mississippi to being sent to Virginia. Since Garland's Brigade were already well organized they were slated to be sent to Virginia to reenforce Lee and possibly constitute a 2nd Texas Brigade in the A.N.V. creating a Texas Division fighting under John B. Hood.
Well we known the story of what happened to Garland's Texas Brigade at Arkansas Post. They were sent to prison camp and instead of being exchanged at Vicksburg, Grant was able to get them exchanged at City Point Virginia. Well Bingo here was Lee's 2nd Texas Brigade delivered directly to him by the Federal Government. With the need for replacement troops in the A.N.V. in April 1863 why didn't Lee keep them? They certainly could not be returned to the Trans-Mississippi by that time.
Everything I have found is that because of the manner of the surrender of Arkansas Post had occurred, these soldier were not trusted and no one wanted them. Especially it seems Hood's Texans because by then they had earn a reputation to uphold. The decision was appearently made to sent them to the A.o.T. to be "closer" to their home Department and that there were more Texans and Arkansawyers in the A.o.T.
It was thought that these Arkansas Post Troops should be broken up and used as replacements in other regiments to Stiffen their Backbone so to speak.
But it was Pat Cleburne who spoke up and said he would take these Arkansas and Texas troops as a whole and make soldiers out of them so to speak. And that is how they maintained their Unit designation in the A.o.T. instead of being disbanded.
As we know that after Arkansas Post the fragments of the 19th and 24th Arkansas and Crawfords Batt. that were not at Arkansas Post, were gathered at Camp White Sulphur Springs and formed into a "Holding" regiment under Col. Chas. Dawson and Lt. Col Hardy waiting until such time as those that were captured at Arkansas Post were to be exchanged back to Arkansas. The Orders in the O.R.'s of May 1863 mentions this unnamed regiment as being unattached at Pine Bluff.
It wasn't until June that this regiment was activated as the 19th/24th Arkansas under the command of Lt. Col. Hardy after Col Dawson's resignation. Hardy's regiment was assigned first to Brig. Gen Daniel Frost Missouri Brigade until after the Battle of Little Rock when it was transferred to Tappen Brigade.
But again back to the main point The Government at Richmond and R.E. Lee has two brigade of infantry delivered to City Point, Virginia in April 1863 that can not be realistically sent back home and they do not keep them. Why? This is the only explaination that I have seen anyone expound on about why they were sent to the A.o.T. and Braxton Bragg and why only Pat Cleburne would have them.
A secondary reason may have been the events that were then taking place in Virginia such as Chancellorsville and Gettysburg may have prevented Lee from taking and reorganizing these troops at that time. But that would not have prevented them from being retained and trained at Richmond for the return of Lee's Army after Gettysburg to Virginia, when they would have certainly been needed, even as replacements.
It certianly seems that the surrender of Arkansas Post, especially after they had repelled the Union land attacks, and after a lenghty review and courts of inquiry of the circumstances surrounding those events, made these men became "Hot Potatos" that nobody wanted and that the general opinion seemed to be that the regiments should be disbanded in dishonor. Certainly Col. Robt R. Garland, a good and honorable professional soldier, took the blame for Arkansas Post and did not serve afterward.