Doesn't support wayland
In wayland the contents of the clipboard don't appear
Is the application running under Wayland directly or XWayland? I think if it is running under XWayland there might be issues with synchronizing the X11 and Wayland clipboards. Also, what DE or WM?
I tried testing it on Wayland. As you described the clipboard content doesn't appear. Unfortunately this is due to a restriction on Wayland which does not allow clipboard pasting from unfocused applications. As with Mac, a workaround is now applied to instead check for clipboard content when the window regains focus. This unfortunately breaks the backgrorund single-word lookup feature which doesn't require you to focus the window at all, but I don't know of a better way to fix this.