Quick first response while I recover from too much food!
This part of his records starts in March 1942, Dunkirk was in May/Jun 1940 so no evidence here of that. The left and centre columns record his postings and the authority for each as he moves from 11 Armoured Brigade to 19th Infantry Training Centre East Lancs Regiment (at Squires Gate Camp Blackpool) in Mar 42 and on to other units (I'd prefer to see more of the record to clarify the remainder).
The right hand columns deal with promotions and start with him being promoted (must have been from Corporal) to Unpaid Lance Sergeant in April 1943 and on the same day to Paid Lance Sergeant (does the job and get the pay (half way to Sgt). Then to Unpaid Acting Sergeant on 13 Jul 1943 (almost there, still paid as a Lance Sergeant). Then the unit find they have made a mistake and they have too many [full] sergeants on the War Establishment (the number of each rank they are allowed to have) so he has to revert to Paid Lance Sergeant on 23 Jul 1943. In December 1943 it all is sorted and he is appointed Unpaid Acting Sergeant on 19 Dec 1943. On the same day he starts to receive the pay of a Sergeant.
In Jun 1944 he reverts to his substantive rank of (War Substantive) Corporal which he was at the top of the story.
The essence of this is that from April 1943 until Jun 1944 he was doing the job of a sergeant and wearing the rank but not being confirmed in the rank so it was relatively easy for his rank to be taken away if he did something wrong. Whether the reversion to WS Cpl (substantive meaning that it was his confirmed rank) was simply an administrative thing, the remainder of the record may show.
Are you able to post other parts, particularly his "Service at Home and Abroad" (a short table) and more of his "Service and Casualty Form" plus his "Statement of the Services" table. To get a full picture it is necessary to read parts of the form together.
maxD