Mission Control is not working after connecting my Mac to a new monitor

Hi there, I am having problem that after connecting my Mac to a new monitor, using Mission Control gesture will trigger both screen. I have set the ‘Displays has separate spaces’ to on mode but it doesn’t help.

This only happens when I am connecting to my new monitor and it didn’t happen before. After this happens, I tried to connect to former monitor that worked well before, but the same problem happened, even for my ipad as second screen. I tried to restart but also didn’t work. 

I am currently using Sonmona 14.6.1. This can't be caused by system update since this haven't happened ever since I bought my mac.

I really wish my second screen stay table while i can switch between different windows on main screen. Any help is appreciated!

[Re-Titled by Moderator]

Original Title: Mission control on monitor