Immich in a browser doesn't seem to have support for Apple ProRAW images, because I can't see their thumbnails or view them at all within the Immich web app. It is for sure only doing this for the Apple ProRAW images, and no others.
I shoot almost everything in raw, so this is a bit disappointing, but I'm sure it's just because no one has discovered this is a bug and report it yet.
I'm on the newest everything. Newest Mac, newest Immich version, and this is still an issue when connecting locally to the server, however it doesn't occur on mobile, at least the one mobile I have, which is probably because it had the images already cached from before it uploaded them to the server, so of course they would have thumbnails on that one device still.