Sharepoint - Master page not updating for just one user account - SharePoint Online

As I think this is the problem with Check-in and Publish problem.

I hope you have Checked-In the Masterpage and published it again. Any changes to Masterpage requires this operation.

While publishing the masterpage or any publishing page you will have two options.

  • Publish as Minor Version(Draft)
  • Publish as Major Version

Major version publish is always considered as final version and it is available to all users(Changes done in masterpages will now reflected to all users)

You can find the point to point difference between these here

So that might be an issue