Yes, all tanks have an over-flow.
Air escaping will go out faster when filling than water will when full and it is over-flowing. The air just moves faster than water. The water, by going slower, will cause the tank to pressurize. Solution is to fill it slower so the water can escape out that little tiny overflow tube faster/as fast as you can pour it in with water hose pressure from the city.
Unlikely it is clogged as the pressure would blow out most things that would clog it.
Most likely you are just filling faster than water can escape.
Find where the overflow tube goes, or yank it off and run new tubing so you know where it goes and know it is not kinked or anything else.
It only needs to be 5 inches higher at its highest point before pointing back down to exit the floor, higher might be better.
As mentioned, if there is a low spot on the overflow tube and water is trapped there then it will take more pressure to blow that water out with the displaced air, but it will move eventually.