I'm not sure, but I believe that when there is a path for water to follow by gravity, rather than capillary wicking, it will flow with gravity. It will want to equalize the water levels in the box and reservoir. And will, of course, soak the soil to whatever common level the water seeks. Should be easy to find out. If it's going to happen, it should happen even before soil is loaded into the box. Just drape the wick cloth over the side of the reservoir, wet the cloth throughout, and watch the water flow out.
I think it will do this whether the high end of the wick in the soil box is higher than water level, so long as any part of it is below the water level. It will only stop when enough water leaves the reservoir to bring the water level below the lowest part of the wick.
If the weight per square foot began to get critical, I think I would make the reservoir wider and have one end of it extend under the soil box so the wick can pass upward from the bottom of the reservoir into the soil. The wick can be a close fit with the opening for it to pass through. Closeness of the fit won't stop it wicking. My wicks pass with contact through the course nylon fabric that holds the soil.That also lets excess rainwater run automatically down around the wick into the reservoir, helping refill it or just running over when it's full.
Consider nylon ropes as wicks. They are very easy to manage when you're loading the soil or if you have to dig and can be wound around to service every level of soil at every point. The are efficient wicks and empty my reservoirs exactly as the soil demands. (Before I forget, if it hadn't occurred to you, you do have to wet the soil, really wet the wicks, in the beginning before it will draw up the water.)
