Even with the newest image containing Nextcloud 30.0.2 the preview of the photos captured with iOS 18 are missing. Funnily enough there is a message that you should not use Imaginary as it has known issues with HEIC files.
The issue is fixed with an upgraded libheif (1.18.0 includes the needed change) and an ImageMagick version compiled against it.
More information about this (in German) under https://decatec.de/home-server/nextcloud-fehlerhafte-bildanzeige-von-heic-bildern-beheben-ios-ipados-18/