Screenshots of 'Active Window' and 'Rectangular Region' areas aren't taking with Spectacle under Wayland.
I have the same problem. You can take a screenshot of the full desktop but not of a specific region. I notice also that when starting spectacle on wayland, the cursor is limited to only one windows and you need to take a screenshot which is done for the whole screen (two if another monitor is attached).
I confirm the issue. Using Spectacle 18.04.2, Plasma 5.13.1 on x86_64 Archlinux, Mobility Radeon HD3470. I can provide more info if needed.
*** This bug has been marked as a duplicate of bug 377157 ***