I ran into a similar issue with thunderbolt and two laptops. Laptop A was being charged by a usb-c dock, I plugged laptop B into laptop A via thunderbolt (for thunderbolt bridge networking). And laptop very slowly drained power in the battery but was running off of the power from Laptop A.

Then I plugged laptop B into it’s own USB-C brick (on the same surge protector), and nothing happened other than it took me a quick second to think if it was okay to do.