EAS does not support Mileage fields.  What client are you using when syncing?  The clients I have tested are successfully syncing the Spouse/Partner name and Anniversary.

The supported contact fields in EAS per MS-ASCNTC, 2.2.2 are as follows:

Standard Android 10 on a Samsung S21.

You can add a birthday, but no anniversary.

If I switch the contact type to phone or Google, then it's possible to add either. 

It sounds like a client issue not providing an option to fill out that particular field.  It is a supported field in EAS and SmarterMail does handle it appropriately.  I personally have a Samsung S9 (Android 10) and the default contact app does not give me an option for that field.  Please verify that the contact app you are using provides that field.  The protocol nor SmarterMail dictates to the app what fields to provide--that's up the client implementers.