This looks like a device issue. Have you tried the app on other phones? Another thing to try is to reduce the number of items in the gallery to see if it is a performance problem.
Which version of your Android Mobile Device?
Which version of PowerApps within your Android Mobile Device?
I have made a test on my side and don't have the issue that you mentioned. The version of my Android Mobile Device (Samsung Galaxy C9 Pro) is 7.1.1, the version of PowerApps app within my Android Mobile Device is 3.18061.12.
Please check if the version of PowerApps within your Android Mobile Device is the latest version, if not, please update the PowerApps app into the latest version, then check if the issue is solved.
In addition, you could also take a try to run your app on other Mobile Devices (iOS Mobile Phone or Windows Phone) to check if the issue still exists.
You could also take a try to re-start your Mobile Device and then try your app again to check if the issue is solved.