Tiamat shouldn't have an avatar to begin with. In 5e, that's greater-deity stuff, and Tiamat is only a lesser deity.
Either they forgot that, or they "forgot" that. However, while mid-edition retcons irritate me, I have grown to really dislike that greater/lesser-deity lore, so eh whatever.
In fact, personally, I think no god should have a true body, only avatars, their soul having become one with a force or philosophy.