Multi-tenant organisation - when a webpart tries to access Graph API, the user is prompted to login

Target SharePoint environment

SharePoint Online

What SharePoint development model, framework, SDK or API is this about?

💥 SharePoint Framework

Developer environment

Windows

What browser(s) / client(s) have you tested

• 💥 Internet Explorer
• 💥 Microsoft Edge
• 💥 Google Chrome
• 💥 FireFox
• 💥 Safari
• mobile (iOS/iPadOS) - Edge
• mobile (Android)
• not applicable

Additional environment details

• latest Edge
• SharePoint Framework 1.21.1
• Node 22.16.0

Multi-tenant organisation - the SharePoint Online instance is access by users from other tenants, via their external member account - automatically provisioned by Entra as part of the sync process.

Describe the bug / error

We have a number of users who are reporting that when they access a SharePoint Page which has a webpart that needs access to the Graph API, they are being pinged to the standard Microsoft login form, prefilled with their external member username (i.e. user.name_domain.com#ext#@tenant.onmicrosoft.com).

Obviously, they can't login with that account, because it has no password.

On one machine, we managed to solve it (for now) by clearing a bunch of cookies. However, on other machines, we tried the same approach, and it makes no difference.

I'm wondering if there has maybe been a recent change to authentication, which is causing token generation for the Graph API to fail, when the user is an external member, rather than a tenant user. I wondered if anything like this had been reported already?

Steps to reproduce

1. Access SharePoint Online page with a webpart that needs access to Graph API (even something very simple/low privilege), with external member access
2. Note that a Microsoft login form appears, with the external member username prefilled.

Expected behavior

Expected behavior, is that the user wouldn't be asked to login - and they will be issued with a Graph API token. This used to be the case until a week or so ago.