Under Wayland only applications that have the keyboard focus can interact with the clipboard (a.k.a. `wl_data_device`). Such constraints are not a natural fit for Wine's current clipboard infrastructure, which uses a separate thread and window in the desktop process to act as the win32-side clipboard proxy for all native windows.
It seems to me that this is mostly specific for winex11 and winemac use cases, but I don't think it does anything if the driver doesn't implement the ClipboardWindowProc callback.
I think you should be able to use a different method for winewayland, for instance use the UpdateClipboard callbacks directly, and if you need to be notified when clipboard changes, wineserver could post internal wine messages to the foreground window too. Note that the foreground window is not very likely going to be the one with focus as it's not kept in sync.