Spectacle always uses “Full screen” area on first launch with print screen key when invoked by Wayland. (If this doesn’t occur to you, and you want a video, I’m happy to provide that. However, then it would probably be in the realm of http://bugs.kde.org rather than here.)
Is this the expected behaviour? Flameshot doesn’t appear to do so, and Spectacle doesn’t when invoked by X.