SharePoint Subsite - Global Navigation Links

The navigation in SPO has changed with the new looks. You can manage the navigation from the site settings -> Top Link Bar. When you create subsites you can define wether or not to take over the navigation bar from the parent. This defines how the navigation on subsites will be implemented and you can change it from Settings -> Top Link bar.

When I create subsites they display in the old and in the new look-and feel.  When you have sites created before this was implemnted then it appears that they don't show up and only display the old navigation, as defined before the change of the new document library looks. So it seems that there was no conversion at that time for the sites created in the old way and you will have to add these to the Top Link bar manually.