Hi FashionDrips - hope you're keeping well!
This isn't a common issue with the Video-player, as most publishers would not have the URLs changed on the mobile versions - we apologize for any confusion or inconvenience.
This issue is specific to how you've setup your mobile site - not using the same URL for a desktop/mobile page means the placeholder you're setting up on the desktop URL won't translate to the mobile URL. They're essentially two completely different pages - we can't expect Ezoic video player to automatically see that it's the 'same page' when the URL is entirely different.
If you chose to keep this mobile setup in place, then YES you would need to setup specific video placeholders for the mobile page URLs, as again they're essentially a completely different webpage from the desktop version.
We hope this helps to clarify, if you need further assistance/have questions please let us know!