How did Promotions work for Drafted Soldiers

My Grandfather was Drafted in 1944 he was a Test Pilot for Gliders in Lubbock before being transferred and got his Technicians badge and then his Aerial Gunner wings and was a Tail Gunner on a B-29 and after being assigned to a CCTS his Bomber Squad. was making their last practice bombing run before going to Japan when their plane went down and the crew had to bail out, my Grandfather landed in a rocky field breaking his leg, after this my Grandmother said "he was put in the supplies for the unit he was in".
That being said I have read that when a soldier got their Technicians badge they were promoted to Tech.Cpl and when they got their Gunner Wings they were promoted to Sgt my Grandfather received and Honorable Discharged as a PFC and I was told by some that when he got hurt and could not perform his duties as an Aerial Gunner or a Technician that he lost his "raise in pay" and was put back to PFC and others have said that lots of Drafted Soldiers were not promoted past PFC even with Technicians badges and/or Gunner Wings and I have read of Aerial Gunners (tail gunner, ball turret gunner, waist gunner) that were PFCs and on this matter i was told that Aerial Gunners on bombers like the B-17 and B-24 were not promoted but since the B-29 was so advanced in Technology that all of the Technicians/Gunners for the B-29 was promoted so someone please help me sort this out
Oh and on my Grandfathers DD214 in Box38 "Highest Grade Held" its blank

 

I hadn't been aware that they suspended voluntary enlistment. Intersting. Did you have any choice of which service you went to if you were drafted at that point?

 

So that would mean that when he got injured they put his rank and pay back to Pfc. because he could no longer perform his duties as a Technician/Gunner or they screwed up his DD214 and Discharged him as a Pfc when from my reading he should have been a Cpl or Sgt

I am Bald I have pulled out all my hair trying to figure this out.................

 

Well from what I have found out from Family Member, Web Surfing and You when my grandfather got his Technicians Badge he was promoted to T/Cpl then when he got his Aerial Gunner Wings he was promoted to T/Sgt and when he was injured and could no longer perform these duties he was placed back at Pfc and was discharged as such

in this Training video from 1943 the instructor says they will receive "Automatic Sergeantsy" upon graduation
but others say they received rank of Cpl and in my research I believe that if a soldier went to aerial gunner school he became a Cpl but if he went to technicians school he became a Cpl and then when he went to gunner school he became a Sgt does this sound about right to you all?