For what it's worth:
With SM V15, Outlook on IOS works for me using IMAP. Last time I tried, it did NOT work on SM V15 with EAS.
On the other hand, SM V15 + EAS (or IMAP) seems fine using Outlook on Android.
I was troubleshooting the IOS / EAS problem with Microsoft, but got sidetracked, and IMAP was OK with the users involved. Perhaps I'll re-visit that issue. Seems to me that, as a Microsoft-licensed protocol, and it works on Android, IOS / EAS should not be a problem.
In the Outlook EAS settings for Android, there is a field for <domain>\<username>
in Outlook EAS on IOS, <domain> and <username> are separate fields, possibly because (I believe) there is no backslash in IOS-speak.