The next surviving muster roll is for the period Aug 31 to Dec. 31, 1862, dated January 20, 1863
The rolls covering the periods March 1 to August 31 are missing. We can then assume that he must have been dropped from the rolls, either because of transfer, discharge, capture, desertion, etc. as he does not appear on the rolls again.
Company I, 14th Texas Cavalry's muster roll dated April 5, 1864 shows them stationed at Lauderdale, Mississippi. "This roll was not made out on the 1st of April as the command was on the march from Demopolis, Ala. to this point (Lauderdale, Miss.) & was made as soon thereafter as practicable."
As far as Robert is concerned, there are just too many units at Mansfield on the Order of Battle and neither appears to have been assigned to another unit that I can find.