I'm experiencing the exact same issue at the moment, it's been on and off for the last 2-3 months, with no real indication as to what's causing it - it's been very random for me.
My only fix is to use a CSS Pseudo element paired with a media query in order to ensure the fallback image appears on mobile. Far from ideal, but it works for the time being.
YouTube banner video not loading on mobile device and fallback image does not show either
in Mobile
Posted · Edited by ANI.agency
Elaboration
I'm experiencing the exact same issue at the moment, it's been on and off for the last 2-3 months, with no real indication as to what's causing it - it's been very random for me.
My only fix is to use a CSS Pseudo element paired with a media query in order to ensure the fallback image appears on mobile. Far from ideal, but it works for the time being.