What if:

• You installed Nextcloud via Docker
• You created files or subfolders in the Nextcloud folder of a user
  • This is also true if you’re restoring files and folders from a backup
• You can’t see the files or subfolders in the Nextcloud web interface

Just run the following command, replacing nextcloud_app with the actual container name of your Nextcloud Docker install:

docker exec -u www-data nextcloud_app php occ files:scan --all

This is working for me, on Nextcloud version 27.0.1.