I can't see the video, but what you are describing sounds normal to me
My thinking is, with my experience, when you turn the return on, the water level in the display goes up until it reaches height for the overflow. That displacement if water from the rear chambers into the display lowers the water in thise chambers.
Now the question is, will you need to add water? Ir are those chambers supposed to run lower?
With a sump, not aio, if all your water level is correct in your sump prior to powering on your return pump, once you power it on, you will see that water needs to be added because the total volume of water has not changed but the volume the water occupies has. BUT once you power the return off, you will have that extra water fill the sump over the water level it operates at, sometimes this is a significant volume and you need to plan for that extra water when pumps are off to prevent overflow.
However, with an all in one, that design wirk has been done for you, so its just a matter of finding out where your operating water level should be. If the water level is supposed to remain higher in the later chambers, then add more, if they are meant to be running lower level, adding more water will cause water to overflow when the pump is off without extra headspace for the water to occupy.
Is the water level in your display up to the overflow line? You could also have this problem if you are not running the appropriate amount of water there. (I used to run way lower in my fresh tanks, and had no idea you could run them nearly cinpletely filled like you have to with a salt tank with return pumps)
