By default, The Witcher does not have a usable depth buffer for Reshade. However, using the Copy Depth Buffer Before Clear option, a usable depth buffer can be retrieved. However, when the setting is enabled the game will flicker between looking as it should and broken fog, shadows, and objects appearing. When its broken, the depth buffer is also improper, while when it looks right, the depth buffer is as it should be. It's not an issue related to effects, as it occurs when the setting is enabled regardless of whether any shaders are enabled. Here's a quick comparison between how the game should look and how the option makes it look:
(These screenshots are old but the issue remains in the latest release).