I found out how the Graphics.RenderMeshIndirect method and the startCommand argument works.
startCommand must be used with commandCount in mind. If, for example, a commandBuffer with 16 rendering commands was passed to the function, and you set startCommand = 8 (that is, you will not draw the first 8 commands), then you need to specify commandCount = 16 - 8 = 8 so as not to exceed the boundaries of the commandBuffer.