In SharePoint Online, I use Windows Explorer to add and manage folders and the data within the folders. Accessing SharePoint via the browser for the amount of data I work with is far too cumbersome. The problem exists when I modify a SharePoint list entry where the column is a lookup field for the folders that I created using Windows Explorer.
I can see the all the folders I created with Explorer in the SharePoint Document Library, just not when trying to select it from a lookup field. I have however, found a solution but it too is very cumbersome, but it's the only way I've been able to get SharePoint lookup to recognize the folder.
If I select the Rename feature for the folders that do not show up in the Lookup field and save it, whether I change the name or not, the folder will now appear. It should be pointed out however that this only works when using the "old" or "classic" SharePoint document library interface. The newer interface Rename feature still does not resolve the issue.
The question then becomes why can SharePoint Document Library see the folder fine but the Lookup feature cannot?