Why does my recently installed app keep downgrading?

Not sure here is the best community for this issue, sorry if it isn’t.

As a developer, I am proposing some promo codes from the App Store Connect website to download a non-released version of my app (available once Apple validated the review).

On some iPhones, the code is working fine: it automatically starts downloading the new version, but then once the download is finished, it automatically downgrades the app back to the previous installed version.

The problem can be reproduced after the iPhone has been restarted or with a new generated code.

Any idea why?

Source: Ios