audio/video sync problem

hello, i was just wondering if some of you guys could help me with this problem.

my youtube videos are fine and in sync everywhere but the sound is out of sync by one second when i use the mobile app. other people's videos are fine when i use the app and i don't understand what's happening.

at first i thought it was settings-related so i asked a bigger youtuber who uses the same program as me to show me her settings and we basically have the same ones.

i tried deleting fcpx's preferences, clearing the mobile app's cache but still have the same problem.

if someone could help me, that would be appreciated!