His service record in Andrew B. Booth's "Records of Louisiana Confederate Soldiers and Louisiana Confederate Commands" gives a bit more detailed information on him (below). A look his service record in the National Archives microfilm (Series 320) might provide documentation that was not copied in Booth's books and further explain his assignment in Louisiana.

Montgomery, Robert, 2nd Lt. 1st Lt. Co. E, 8th La. Inf.

En. June 19, 1861, Camp Moore, La. Present on Rolls to Aug., 1861. Roll Sept. and Oct., 1861, Absent without leave. Rolls Nov.,
1861, to Aug., 1862, Present. Rolls Sept., 1862, to May 14, 1863, Absent, wounded, since Sept. 17, 1862. Rolls May, 1863, to Dec., 1863, Absent, detached service, May 13, 1863. Roster dated Camp near Raccoon Ford, Va., April 6, 1864, Elected 1st Lt., May 13, 1861. Promoted 1st Lt., April 6, 1863. Record copied from Memorial Hall, New Orleans, La., by the War Dept., Washington,
D. C., June, 1903, Disabled by wounds, Sharpsburg. Transfd. to Louisiana.