ProtectedViewWindow.Workbook returns Nothing

Question

I have a VBA macro that suddenly stopped working after 3 years of functioning.

In my script - I address ProtectedViewWindow workbook and unlock it if it matches my criteria.

But now Application.ActiveProtectedViewWindow.Workbook returns 'Nothing' even though workbook clearly exists and is protected. ProtectedViewWindow has all other fields populated correctly (SourceName, SourcePath, Caption etc). But not the workbook reference.

Why did this stopped working? Is that somehow related to a security update?
And is there a work-around?

I'm on Microsoft® Excel® for Microsoft 365 MSO (Version 2206 Build 16.0.15330.20246) 64-bit