Office 365 has 15 attributes, called "Custom Attribute 1", "Custom Attribute 2" ... "Custom Attribute 15".
Those attributes for some reason are not visible from Azure AD.
I tried https://graphexplorer.azurewebsites.net/#
I tried power shell. They are just not there at all.
However, all other attributes, like Department, are available in both places (Azure AD and Office 365) and their values are synced properly.