I see a subsurface in WAYLAND_DEBUG being used that's definitely associated with the preedit, though for whatever reason I'm unable to get anything visible on screen.
I'm personally building a native-PE ime based on libhangul. Probably the same problem I had with the wayland driver. Hopefully the attached patch will help.
[fix_wayland_ime_ui_window.diff](/uploads/2cdf62ea84efcd1f40bb3e75b98a7565/fix_wayland_ime_ui_window.diff)