I am glad you have or appear to have solved your problem but I fear that using the lower temp may not have been the real solution to your problem. I see no reason why a Jobo reel and tank or 38C would affect 6x12 and not any other 120 film in other sizes such as 6x7

The chemistry does not know that you have changed the temp and the Jobo reel doesn't know that it has 6x7 in it so behaves properly but chooses not to do so with 6x12 in an unholy alliance with 38C. It is the same 120 film after all

There has to be another reason why this has worked