Hi Jana
Unfortunately Safari for iOS is notoriously bad at dealing with web content in iframes or framesets, leading to issues like you describe.
You could try installing the adapt-iOSScrollFix plugin, that might sort it out. Alternatively many choose to force the course to create its own new window so that there's no iframe involved - there's quite a bit of discussion about this here. We also built an extension to help with this which can be found here.
Equally it's possible to modify Moodle so that it doesn't use an iframe, you can find more info about that here.