I get results by deleting the actual HLOD layer from the content browser, the Delete from the Build HLOD in the Build menu is a bit …hmmm… doesn’t work to well for my old dodgy map and my new one too now.
First Make a new HLOD layer in the content browser,
then delete your existing one, replace reference with the new one
don’t save at the save content from the next window.
After that I usually
Build Landscape, save project, Build HLODs, save content if prompted.
Been doing this everytime I build HLODs, maybe 15 times on one & the other is at NewHLODLayer9.
Have tried just building HLODs without total layer delete every so often but I just get Validation Fail.