Turn on view HLODs in the Outliner via the Editor Preferences
type HLOD in the outliner filter.
There are a couple of rogue props generating HLOD layers, but because they are not referenced by your project it makes them invalid.
Any HLOD references there that are not your HLOD layer name are either your old HLOD layer empty(or full) folders or they are being generated by certain Fortnite props.
For instance if you are using a few types of Asteria Trees and most of the Signs from Mega City you will see AsteriaTreeHLODLayer and NeoncityblahblahHLOD or something greyed out.
Select These and Force Load, then double click on them to see which props they are the HLODs for
Next drag your HLODLayer into each props Default HLOD layer slot.
This will override it and make it use your HLODLayer
When your finshed DeleteHLODs from the Build menu and manually delete all the HLOD folders in the outliner, double click your level thumbnail to reload map and BuildHLODs
Repeat if you didn’t get them all first go