MS Word changed a folder to a Word document/folder/hybrid

Not sure how this happened, and I couldn't find a mention of it in the forums.

One of my folders suddenly changed from a folder (with a blue folder icon) to something I've never seen before: a Word icon that fronts a folder. The new folder has the same name, and everything inside it remains the same -- but I can't move new items to the folder.

Has anyone seen this before? Any suggestions? TIA.

iMac / Retina 5K

OS 10.13.1

Word for Mac 15.41

View inside the folder.