The settings sometimes get messed up by updates or other apps.
You could check to see if it's also true when viewing Photos or Music folders but more to the point, it sounds like an error in the registry.
This site suggests a couple of fixes, one of which worked for some.
The solution that worked in this case:
Open the Explorer registry settings and turned off "Web View":
WebView = 0x00000000 (0)
Closed all instances of Explorer and re-opened a folder (no reboot).