Tried all the suggestions...
... Options, check box off. Nope
... Cleared thumbnail cache... Nope
...
Finally... I went into onedrive and made sure everything was completely synced (all folders with solid green dots and white check mark)... Then I uninstalled OneDrive and magically, my thumbnails reappeared immediately.
Then I re-installed OneDrive using the same folders and the thumbnails are still showing correctly for View-->medium, large and XL icons. Thumbnails show correctly for all 3 sizes.
Only took 10 minutes to uninstall/reinstall. Frustrating though.