f13a-6c3a Hi there! I can understand that this is perhaps confusing, but it's not a bug.
On Stock OS, Google Camera has the themed icon because Pixel Launcher hardcodes those themed icons for a lot of their apps into their launcher, instead of doing it the proper way within the app.
Hopefully they will eventually adopt this for all of their apps. Until then, there's not much that GrapheneOS (or anybody else) can do about it.
Further reading: https://twitter.com/MishaalRahman/status/1570448481520652288