Hi @Anonymous ,
I agree with @CarlosFigueira 's thought almost. I have also made a test on my side (on my iOS mobile device and web browser), and don't have the issue that you mentioned.
The screenshot as below:
Set the OnSelect property of the Gallery to following:
Navigate(DetailScreen1, ScreenTransition.None)
Set the OnSelect property of the ">"(Navigate) button or other controls within the Gallery to following:
Select(Parent)
Please take a try to turn off/disable the "Improved app rendering (final validation)" option within Advanced settings of App settings of your app, re-publish your app, then check if the issue is solved.
Also please check if the version of the PowerApps Mobile App in your Mobilde device is the latest version, if not, please upgrade it to the latest version, then run your app again, check if the issue is solved.
If the issue still exists, please consider take a try to uninstall your PowerApps Mobile App from your Mobile device, then re-install it, try your app again, check if the issue still exists.
Best regards,