Grounding Arduino UNO with external voltage source
The circuit in the image shows the plan of the Arduino Uno, as you can see the GND pin under AREF and in the GND under the Power pin VCC are connected.
you can double-check this with a tester and figure it out.
so yes, they are internally connected.
now, in your breadboard you have connected GND to VIN, that's the problem, Vin is an input voltage that is regulated to supply 3.3V and 5V.

So remember to share grounds with external voltage supplies (DC-DC), that set the circuit to the same reference.