Playback issues on iOS and Jellyfin App

[Been Reported]

Hello, I was wondering if anyone could help with some issues I seem to be having.

Firstly my setup is Ubunutu 22.04 with Jellyfin 10.9.11 and I use FFMPEG to convert any MKV or AVIs etc that come onto my server, my issue I have is specifically on iOS and the Jellyfin app content doesn't play, but on Android and the web client it does. Here's what my numerous devices throughout the house return with example.mp4

Android TV App -  Player Error Encounter, Will Retry
Android TV Web Browser - Video Plays
Android Phone App -  Player Error Encounter, Will Retry
Android Phone Browser / Web App - Video Plays
Desktop on Chrome and Mozilla - Video Plays
Laptop on Edge and Chrome - Video Plays
iPhone App -  Player Error Encounter, Will Retry
iPhone Browser - Black Screen
iPad App - Player Error Encounter, Will Retry
iPad Browser - Black Screen
Fire stick App - Player Error Encounter, Will Retry
Fire Stick browser - Video Plays

I have done some googling and found that iOS specifically is picky with audio codecs and only works with acc, but then I do FFMPEG -i example.com it states it is AAC, here's my current FFMPEG set up

Code:

flock -n /var/lock/ffmpeg.lock find /jellymedia/ -name "*.mkv" -type f -exec ffmpeg -i {} -preset veryfast {}.mp4 \;

If you're not familiar with find then simply the FFMPEG is 

Code:

ffmpeg -i example.mkv -preset veryfast example.mp4