External guest users now see Classic view in document libraries (was Modern)

We are seeing a recent change in behaviour (last ~1–2 weeks) affecting external/guest users only.

Context:

• Sites are created from an approved SharePoint Communication Site template
• Tenant and site are configured for Modern (New) experience
• Document libraries were previously rendering in Modern view for external guests
• Internal users still see Modern view as expected

Current behaviour:

• External/guest users now consistently see document libraries rendering in Classic view
• The library shows “Exit classic experience”, but selecting it does not switch the library to Modern
• This occurs even when:
• Accessing via the site home page
• Navigating via the site menu
• Using a newly created site from the same template
• Testing in InPrivate/Incognito
• Library settings are explicitly set to Default experience = New experience
• SharePoint Admin Centre is set to New experience (no Classic defaults enabled)

Important observations:

• This behaviour did not occur during earlier testing using the same template and permissions
• No template changes have been made
• Reproduces across multiple sites
• Reproduces only for guest users (internal users unaffected)
• The issue persists even after refreshing, changing views, or setting the library experience explicitly to Modern

Question

Has there been a recent change or regression in how SharePoint Online renders document libraries for guest users, particularly when accessing libraries on Communication Sites created from templates?

Specifically:

• Are there known issues where guest sessions are forced into Classic rendering despite Modern being configured at the tenant, site, and library level?
• Are there any recent changes (late 2025 / early 2026) affecting guest rendering or Classic fallback behaviour?

Any guidance or confirmation would be appreciated, as this represents a change from previously working behaviour and impacts external user experience.