IMO (dubious) 1.2 = 0.6 for percentage drop 24/12. After that we play with V=IR where V is twice (but the same equivalent). We have I being half with 24 vs 12. So R must be way higher in absolute numbers, but it must come out even for same percentage V.
He used the same wire so its ampacity is the same (at 12v). But amps are half so you would expect less of a voltage drop but that would be in absolute numbers staying within the same voltage (12v)
It seems like something is wrong where if the wiring/batts can do 0.6 drop at 12v at 100 amps, it should do better (less than twice) at 50 amps 24v, but he gets twice, so something needs fixing--loose connections?
Or as mentioned is it peculiar to the battery bank being 6s in series instead of 12s in parallel or some such thing?