This is specific to the [b]Active Window[/b] capture mode. All other modes appear to be unaffected. Active windows that do return a capture are some of the default KDE tools like Konsole and System Settings. Failure examples include: System Monitor, Firefox, Dolphin. I've tried via the Spectacle user interface with time delay and click options, I've also tried using a couple of Global Keyboard Shortcuts from system settings.
PS: I wasn't able to set the version number above. Here's what Spectacle reports on the bug report window: Version: 16.12.3 - Orderly Transfer of Power OS: Linux (x86_64) release 4.10.0-19-generic This is an up to date Kubuntu 17.04 installation.
Hmm, can't reproduce with Spectacle from Plasma 5.9.5 in Kubuntu 17.04 or 5.11.1 in KDE Neon. Are you still able to reproduce this?
I strongly suspect this is a duplicate of 390652. If not, please comment and we'll reopen. *** This bug has been marked as a duplicate of bug 390652 ***