SharePoint IOS app diverts document library to the OneDrive IOS app, which does not show the library

Thanks to Sharegate's help, I've got to the bottom of this. When you use Copy Site Objects to migrate a library to a modern Team site on SharePoint Online, it will not copy the DispForm.aspx, EditForm.aspx and Upload.aspx. You can check using SharePoint Designer 2013. Without these, not only can you not edit the properties of an item, the OneDrive iOS app cannot see the library.

On the Sharegate migration tool troubleshoot, it suggested you to enable Custom Script on the SharePoint Online environment. There is an option on the SharePoint Online Admin settings. However, that is not enough. You have enable this using PowerShell from the SharePoint Online Management Console.

This is the instruction from Sharegate:

1. Download and install Sharepoint Online Management Shell
2. Open a Sharepoint Online Management Shell prompt
3. Run the two following commands: Connect-SPOService -Url https://yourdomain-admin.sharepoint.com/ Set-SPOSite -Identity https://yourdomain.sharepoint.com/sites/YourSite/ -DenyAddAndCustomizePages 0
4. Try to remigrate.

It worked perfectly! The forms are now in the migrated library. The OneDrive iOS app can now see the library and its content.