Is this a case of ground loop?
Not a ground loop. But a short circuit:


Always use fuses with batteries.

Instead, you can make it like this, so that there is only one common ground:




Drawback is unbalanced load on the battery. Meaning the left battery depletes faster, possible damaging it when the set is deep discharged. I recommend a battery balancer, or a 24V-12V power supply instead.