Move message doesn't show folder tree

Using the move message button in the toolbar, the dropdown list doesn’t show a nice folder tree, but instead a horrid, very hard to read flat text list of all the folders:

Is this expected? (In which case, yuk).
Or do I have a bug? (In which case, how do I solve it?)

Other places in the app, like the folders list in the sidebar under each account, do properly use a tree structure. So this seems odd.