I think it’s possible that batteries don’t charge over jumper cable and the jumper cable just allows charge to be drawn through it.
Your food vendor cart had an empty battery. The minifridge tried to get electricity. It went to all the batteries in the cart asking for charge and they all said no. Then it went to the jumper cable and asked the solar car. The car went to all its batteries and found charge to give to the vendor cart. The vendor cart battery, however, doesn’t ask for charge, so it never gets it.
I don’t know what would happen if the batteries on the car are at 100% and the solar panels make charge though.
I think that’s how it works. I think you’re right, though, that batteries should equalize when attached to each other.