My team has had an issue with PowerApps when viewed on mobile. When you have content on a screen that can be scrolled (i.e., gallery or form), you are left with a double-scrollbar when viewed on mobile. One scrollbar is for the control, and the other is for the web page itself. Would it be so difficult to make PowerApp web pages fit to the mobile window, instead of having the non-user-friendly wiggle room? It can cut off controls/data near the bottom of the screen and makes it feel messy.
Could you please share a screenshot about the issue within your Mobile device?
Further, do you run your canvas app in web browser in your Mobile device rather than PowerApps Mobile App?
Based on the issue that you mentioned, I have made a test on my side, and don't have the issue that you mentioned. The Scrollbar of Gallery control shows well in my PowerApps Mobile App, and the Scrollbar of screen page do not show up.
Please consider run your app using PowerApps Mobile App in your Mobile device rather than use browser. If your canvas app is not designed as Responsive Layout app, when you run your app using browser on Mobile device, you may face rendering issue on your side.
In addition, please also consider turn off the "Scale to fit" option within Screen size + orientation settings of App settings of your app, then re-publish your app, check if the issue is solved.
If the issue still exists, please consider configure your canvas app as a Responsive layout app through the following article:
Perhaps the screenshots will help clarify the issue. Also, I'm using the browser (Safari on iPhone 11 Pro).
It's not necessarily about the scrollbars showing, it's the fact that the PowerApp has so much wiggle room on the screen. If there is a button or other control near the bottom of the screen, it may not show at all, or be cut off. If there is a gallery/form on the screen, the user may attempt to scroll the page and end up scrolling the gallery, or vise-versa. Let me know if I can clarify the issue further. As far as I can see, it's not a bug, just an unfortunate design of PowerApps on browser.