That symbol / trope goes back to ancient times. Not a phoenix, but a water bird "Pelican" something like that. It's on old arms and drawings by "Durer" That's how old it is.
Myth goes like this:
Bird is able to separate milk from poison.
Sometimes it's shown feeding babes and living on a high crag / rock.
If I find the images I'll post with tag Water Bird.
The vesica pisces above the brid is well known.
Look at the legs, It's not a phoenix. Phoenix are red or orange. This is white.
maybe connected with lineage of murdered King [J]Ises?