I have the same problem with hex and i end upsaving a small screendump  image of anything i want to keep.

P9 and PP12 has a scene library built into the programme which produces thumbs (just like any other library, so long as you save the scene within the programme to the scene library it will generate a thumb) 

If you are inclined to upgrade from time to time then this will eventually solve the problem.

 EDIT - Ive just noticed your other post regarding installing p9 error so assume you have just got it , you should be able to from now save as above and see a thumb, the old scenes you can (if you want ) reload but then resave to the scene library so you dont go through all that again at a later date, then delete the old scene file.

Tools:- Win10, Dell XPS8900, ZBrush, Marvelous Designer 11, Hex 2, PSP8. PSP 2019 Ultimate, DAZ Studio, Affinity Photo, Affinity Designer, Filterforge 11, flowscape,  Classic UVMapper, and several headache tablets.