the link you provided is helpful, but it "holds" only once: have to do it every new import.
there is an easier workaround: select all black thumbnails and then "read the metadata" from the files.
I hope this contributes a bit, may be forcing metadata read automatically when importing can be arranged.
- Turn off Apply auto tone adjustments under Lightroom preferences
- Restart Lightroom and then import images again.
Yes, that did it.
You are Welcome, Glad to help.