When you say that you "added the folder", are you referring to theC:\Users\UserName\ArcGIS\My Surveys\Maps folder?
What happens when you attempt step 3 listed in the following linked resource page?
n the Survey123 field app, select Menu > Settings > Map and browse to your ~\ArcGIS\My Surveys\Maps Map Library Folder.
Any survey that has includeLibrary set to true (for additional details, see the last section in this topic) will now include any maps stored in this folder in the list of available maps.
Be interested to know if you resolved this? I've also been having issues trying to get an offline basemap to work on Windows 10 s123 field app. So far trying all methods except for loading 'media' folder. I can get it to work from my maps library on s123 Connect but not the device.
Going to Settings\Map and Browsing for the library often does random things like defaulting to a Survey123.exe folder and not the actual folder I picked.
The same basemap & survey downloaded to an android device works fine.