I have made a test on my side, and don't have the issue that you mentioned. The version of my iOS device is 11.4, the version of PowerApps Mobile App within it is 3.18093.32.
Please check if the version of the PowerApps Mobile App within your users' iOS device is the latest version, if not, please upgrade it into the latest version, then check if the issue is solved.
You could also consider take a try to ask your end users to uninstall PowerApps Mobile App from their iOS device, then re-install it, check if the issue still exists.
For the issue when running an app within IE (IE11), please make sure your end users have turned off "Compatibility View" option within their IE browser. if the issue still exists, please take a try to clear cache of their browser, then check if the issue is solved.
Note: Some functionalities within PowerApps are not supported within IE browser, please run apps within Chrome to get better performance.
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.