Before posting my question, videos in MP4 format did not work. Later, I re-installed UT with the graphic installer (without erasing the user data). So I tested an MKV video, and it worked ! So I re-tried MP4 videos and all them worked !

@TheKit Your fix has been applied on surya kernel by @ywmaa. Users have reported on Telegram that UI freeze are gone.

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

@eheintzmann Yes now it works. Is ubuntu tocuh 20.04 not yet available for the poco x3 nfc?