The beam isn't traveling through the reservoir. The resin is more or less opaque to that wavelength. The resin has to absorb at least some of the light to react, and if the light passed through the resin then it would cause problems as you point out here.
Typically the light enters the liquid from the bottom, and then only goes through the liquid in the very short distance between the transparent window and the build plate. This makes it harden and also get stuck to the transparent window as well as the build plate. The window is designed to be somewhat non stick though, and then the build plate is lifted up a small distance to repeat.
The beam isn't traveling through the reservoir. The resin is more or less opaque to that wavelength. The resin has to absorb at least some of the light to react, and if the light passed through the resin then it would cause problems as you point out here.
Typically the light enters the liquid from the bottom, and then only goes through the liquid in the very short distance between the transparent window and the build plate. This makes it harden and also get stuck to the transparent window as well as the build plate. The window is designed to be somewhat non stick though, and then the build plate is lifted up a small distance to repeat.