For information, before closing this thread :
The flickering issue has been fixed in 20.04 devel channel :
https://gitlab.com/ubports/porting/community-ports/android10/xiaomi-poco-x3/xiaomi-surya/-/commit/9d36e8d5986b2bf21afed7feff46baaa7c7535c9
Thanks to @_Joao_ and everybody on pocox3 telegram channel for their help