Could you please share a bit more about the formula you typed within the OnSelect property of the "Button"?
Which mobile device do you use? Android or iOS device? Which version is your PowerApps Mibile App?
Further, is there any error occurs when you run your app using your Mobile device?
Based on the issue that you mentioned, I have made a test on my side (PowerApps Mobile App version -- 3.19053.21, my iOS device version -- 12.3), and don't have the issue that you mentioned. Please check if the DisplayMode property of the "Button" has been set to following:
Please consider run your app in your web browser, then check if the issue still exists. Also please check if the version of your PowerApps Mobile App is the latest version, if not, please upgrade it to the latest version firstly.
Also please consider turn off the "Improved app rendering (final validation)" option within Advanced settings of App settings of your app, then re-publish your app, check if the issue is solved. Please also consider take a try to uninstall PowerApps Mobile App from your mobile device, and then re-install it again to fix this issue.
Community Support Team _ Kris Dai If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.