Have just moved from Owncloud to Nextcloud, and the NC Windows client is showing the default file icon as a badge for files and folders, instead of the normal sync/syncing/error badge:
However, this only appears in File Explorer itself. When using shell dialogs (open/save), the correct icons are displayed.
You probabaly still have the ownCloud client installed?
There is a crazy artificial limitation on windows as to the number of allowed overlay icons.
Had it, but removed it first. Perhaps an explorer.exe restart is needed.
Yup! That fixed it! 
windows so Iām not surprised a restart is required 
Glad it is solved.