The reason is bit complicated.
Previews are provided by /usr/share/thumbnailers/ and kio-extras thumbnail plugins.
The plugins are used by default, but the “image” one does not explicitly declare supporting heif/webp because their support might not be present on the system (i.e qt image plugins), despite it might well work.
Here it seems your /usr/share/thumbnailers/ are not working and deactivating them made their preview be rendered by the default image plugin Images (GIf PNG, BMP... in the file type list.
tldr; Check your thumbnailers in /usr/share/thumbnailers/ work and their dependencies executables are installed, or removes the concerned files.
If you tried to launch dolphin from the command line you might have got some output hinting this way.
audiothumbnail and imagethumbnail: Mention supported mimetypes explicitly (b61847f4) · Commits · Network / KIO Extras · GitLab will fix the png issue in 24.12, kio does not detect image plugin supports png.