I am sharing some tips, issues to other developpers:
-1- Looks like "Classic" theme should NOT be used. However, it is the one selected by default. Should switch to "Use Device Default "
-2- Check that the Application is not using a "not yet supported" feature. Check here: https://doesappinventorrunonios.com/ I faced this pb using MakeInstantFromMillis
-3- Check you are not using extension
-4- Make sure you create a mobile provisioning profile, see: Building Apps for iOS with MIT App Inventor
-5- Scroll not working for full screen. Workaround (Thanks to @Anke ) is to use VerticalScrollArrangement see: Scrolling Screen Issue in iOS - #12 by Anke
When switching from Companion to built App .ipa, other limitations are arising
-A- Screen Initialize should NOT be used (see: Screen Initialize not working with built iOS App .ipa. Timer should be used to initialize the App
-B- Consequence of the above: Layout is not initialized according to values provided in Designer screen. All layout info (size, position, ...) must be done in Blocks with the proper instructions.
Note that a number of these things are bugs in the build system that we will work out. I wouldn't recommend making significant changes to your apps for iOS builds given that this is still beta software.