Call forwarding schedules are created based on a specific phone number. However, since the Grasshopper desktop app is not tied to a specific phone number, it is unable to respect specific call forwarding schedules. Once you're logged into the desktop app, we'll always try to reach you there.
We are working on a fix for this soon. In the meantime, if you do not want to receive calls outside of your call forwarding hours on the desktop app, you'll need to turn on the "Do not disturb" setting or log out of the desktop app altogether.