Thank you so much for this!
Finally found the solution.
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Open the old Control Panel; type into the Win10 Search Field "Control Panel" and then click the result to Open it. Make sure the Categories option is set; then click on Appearance and Personalisation; then File Explorer Options.
A new window will open -- click on the View tab.
The first choice "UNCHECK" Always show icons, never thumbnails.
Thank you so much! This issue happened to me shortly after the 1809 cumulative features update, so it seems the bug is still not fixed!