I have tried ressetting the views and starting Outlook in safe:3 mode. Still when some of the user created folders are clicked nothing is in them. Sometimes switching back and forth with other folders the empty one will repopulate, but not always. There doesn't seem to be any problem with the OWA as that is where they can be found when the folder doesn't repopulate in Outlook.
This video shows how to quickly and easily add an email signature for all users on Exchange 2016. The resulting signature is applied on a server level by Exchange Online.
The email signature template has been downloaded from: