SUMMARY
On KDE neon user edition with all updates installed, all support for HEIC/HEIF images is gone from KDE applications after a recent update - neither thumbnails nor opening of HEIC files work.

STEPS TO REPRODUCE
1.  Open a folder with HEIC images in Dolphin
2. Try to open one of the HEIC images with Gwenview

OBSERVED RESULT
New HEIC images do not have thumbnails in Dolphin, while older files, before the bug occurred, do, as do other file types (JPG, MP4). 
HEIC files do not open in Gwenview, which states "cannot display documents of type image/heif" as an error.

EXPECTED RESULT
Thumbnails are generated and displayed for all image and video files, including HEIC files.
Gwenview displays the HEIC file.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: KDE neon 6.0
KDE Plasma Version:  6.1.5
KDE Frameworks Version: 6.5.0
Qt Version: 6.7.2

ADDITIONAL INFORMATION
It used to work. I cannot pinpoint the exact update that broke HEIC/HEIF support, as I do not work with the pictures every day, but for images dated 12 August, thumbnails were still generated.

I tried several other image viewers besides Gwenview and it appears others with KDE/Qt dependencies are affected as well. Non-KDE software (e.g. XFCE's ristretto image viewer) displays the files without issue, as do the phones on which they were originally taken. So the files as such are fine.

To exclude interference by third-party software sources: I only have two ppas enabled (tlp & nextcloud-client) the rest are simply Neon's out-of-the-box sources.