XFCE – Laptop monitor is not completely off when resuming from suspension (using external monitor)

I am using Xubuntu 18.04, XFCE 4.12.2 on a laptop with an external monitor. I only use the external and disable the built-in laptop monitor. When I suspend a session and resume, the built-in monitor is turned on, then off after I logged in, but the backlit light (of the built-in monitor) is still there. If I want it to be totally off, I have to go to Display setting, turn it on and off again.

I don’t know if this is a bug or am I missing something?

Thank you in advance!