I was having this issue as well with a series of .doc files in File Explorer. I found .doc under Default Programs, but the only Application in the list was Adobe. From File Explorer I then tried dragging the .doc file onto my Word shortcut in the taskbar, which pinned the document to the list. Then I clicked to open in Word. Back in Default Programs, I now had Word in the list next to Adobe, so assigned .doc files to open with Word. This converted (or possibly restored) all the .doc files to Word documents.