When 319s first arrived, there was no agreement for 12 car working. Up until May 2019, max of 8 car working permitted. In much the same way as some posters here are quick to argue that agreements at individual TOCs aren’t precedent for other TOCs, neither are agreements within TOCs.
I am not disagreeing with precedent somewhere does not set agreement elsewhere.
But you are wrong to say "up until May 2019 max of 8 car working" w.r.t. 319s.
Connex South Central operated 12car 319s. And Connex were no more from 2001.
In the period when 319\2 were the all day every day Victoria Brighton fasts, there were in morning peak two Up Brighton Victorias that were 12car 319s; in the evening peak two down of one each from London Bridge and Victoria to Brighton. There were formed by attaching 319\0 to 319\2; IIRC one working was 2 x \2 + 1 x \0 and the other 1 x \2 + 2 x \0.
I repeat, not attempting to suggest that just becasue CSC did that anyone else could do it, then or later, just pointing out that 12car 319 occurred almost 20 years before 05\19.
Anyway, way off topic for this thread.
Are we actually sure these short formed EMR 360s are down to a unit shortage - or to a staff shortage ?
If - and I am unsure - the agreement is "guard" in each unit, even on an 8car, then if there are not enough staff to cover all that, then they have to run a short train. I just find it hard to believe that given Desiro and Siemens high reputation EMR can't turn out 14 (to cover 7 8car) from 21 units.