Hello I have the latest nextcloud android client 3.18.1. In my configuration I don’t use subfolders when I synchronize content from my mobile phone so all the wideos and movies land in the same subfolder. I have dozens of files there.
When I take 5 new photos wait a few minutes until I know that previews are done on the server side open Nextcloud android client go to InstantUpload and Camera folder it shows the wheel spinning for about a minute or two. During that time the pictures icons are without previews. After those minutes the preview is shown.
Such behaviour can be a stopper for many people using the app on telephones. The preview for new files should made as fast as possible in the background. Additionally previews for few latest files should be cashed by the application in the background so that when user enters the app, the previews where present and available for better user experience.