In Windows 10, version 1809 and Windows Server 2019, File Explorer may not display the offline file icons correctly. An offline file should be display by having an overlay that shows an "x" character in the lower-left corner of the file icon. Instead, the offline files may be displayed by having a "normal file" icon. This icon is indistinguishable from a non-offline file.
This issue occurs because of a change in the shell that causes Windows to fail to detect files as offline file. Therefore, File Explorer does not show the icon overlay.
To work around this issue, use a Universal Naming Convention (UNC) path to open the folder. For example, use \\server\share\path.
This method works if "\\server" refers to the local computer or a remote computer. By using this workaround, you can verify that the file in question is actually offline.
Microsoft is working on a resolution and will provide an update when a resolution is available.