Provisioning works correctly by sending the UPN of the Sponsor. However Attribute Sync repeatedly triggers as the aggregated value is obviously different.
A separate Graph API call with the same permissions returns a full record:
Yes — this is a common issue with Microsoft Entra ID sponsor aggregation in SailPoint ISC.
ISC currently aggregates the sponsor object as a serialized JSON fragment instead of resolving userPrincipalName, which causes perpetual Attribute Sync drift.
The issue was raised with SailPoint, accepted as a bug and a fix was delivered last week (although not in release notes). See the use of sponsors.users account schema attribute as described in the updated Configuring Sponsors for Guest User (B2B) and External Member (B2B)