Weird pointer focus behaviour with `xdg_popup.grab`

The xdg-shell specification says the following about xdg_popup.grab:

During a popup grab, the client owning the grab will receive pointer and touch events for all their surfaces as normal (similar to an "owner-events" grab in X11 parlance), while the top most grabbing popup will always have keyboard focus.

However, I observed the following behaviour in Weston 11.0.0 when pressing the left mouse button while over a surface, in response to which an xdg_popup is created and its grab request is called, and then the mouse is dragged over the popup and released again:

• the pointer focus leaves and then almost immediately re-enters the parent surface
• as soon as the mouse is dragged above the popup, the pointer focus switches to the popup surface
• when the mouse button is released, the appropriate wl_pointer.button event is emitted

In my understanding of the specification, I would have expected the following behaviour instead:

• the pointer focus remains with the parent surface (no leaving and re-entering)
• when the mouse button is released, the appropriate wl_pointer.button event is emitted
• the pointer focus switches to the popup surface