Hi,
I have a case where a few users still get the classic document library UI, even though I've set the "SharePoint Lists and Libraries experience" to "New experience (auto detect)" in the SharePoint admin center. I've also set the "List experience" to "New experience" in the document library settings -> advanced settings.
I've also tried to use the "default experience set by the administrator" in the document library settings, but still no effect for those users.
Any suggestions?