Nextcloud version (eg, 20.0.5): 21.0.1
Operating system and version (eg, Ubuntu 20.04): Ubuntu 20.10
Apache or nginx version (eg, Apache 2.4.25): Apache 2
PHP version (eg, 7.4): 7.4
The issue you are facing:
I have installed a fresh copy of Nextcloud, and configured ImageMagick to support HEIC files. When I generate previews, for a collection of JPG and HEIC files, I can see the previews (which are jpgs) in the preview folder. I can see thumbnails for all images in the Photos View of the browser, or under Media in the App.
However in the Files View I only see previews for JPG files, but none for HEIC files.