It does sound like you're talking about the small-ish hose from the tank to the radiator. But just in case, it's not something larger by any chance, is it?
If just the small-ish hose from the tank, then like he said, the caps are designed to allow backflow when cooling down and the pressure is going down inside the radiator.
The old ones used to just spit fluid out on the ground every time you'd go through a large enough heating and cooling cycle. Eventually draining the radiator.
Almost hard to remember now, how often we used to have to check and top off our coolant. Nowadays, you just look in the tank to see if it's still between the lines. Nice.
With the reservoir, and with it's hose below the fluid level like was said, when that pressure changes there is some fluid to suck back in. Instead of just air.
It's all good. I've retrofitted a lot of older rigs because of the benefits. It was simpler back when all we had was a drain hose. But imagine that tiny little bit of coolant multiplied by the number of times a vehicle is driven, multiplied by millions of vehicles! Glad that's not on my streets anymore.
Paul