I can confirm this bug on Lubuntu 23.10.
Previously, on Lubuntu 23.04, I could view HEIC images with all image viewers,
using heif-gdk-pixbuf and libheif-examples.
Now, on Lubuntu 23.10, I cannot view HEIC images any more with the following
image viewers:
NOT WORKING image viewers / graphics sof
At least on my computer, I think the source of the problem is not in
GIMP, because it also effects other viewers and even imgmagick.
I have a bash script for mass conversion of jpg to heic. This shell
script uses "heif-enc" and after the upgrade to Lubuntu 23.10, it's
still working like before.
H