If a KAlarm goes off while trying to select a rectangular region of a screenshot for Spectacle, it becomes impossible to exist the rectangle selection screen. Pressing 'escape' or 'enter' does nothing. The interface isn't frozen; the user can keep selecting rectangles forever if desired. But to get out of the rectangle selection interface, I had to kill Spectacle from a different TTY.
I tried this on the master branches of Spectacle / KDE Frameworks and I wasn't able to reproduce it.
https://drive.google.com/open?id=108fY8_jTR69jk54zvBOSr74CbbwXf14L Video of one of the reproduction attempts.
Does this still issue still reproduces on your machine using the latest version of Spectacle and Frameworks?
You did the right things to try to reproduce it. I don't know why you didn't experience the same issue. I just reproduced the issue with my current configuration and a clean configuration from a new user using the latest Archlinux packages as of yesterday: Spectacle 18.12.2, Frameworks 5.55.0, and Qt 5.12.1
After reading your comment, I tried to reproduce this on another machine and surprise, it reproduced. After rebooting my main PC, I was able to reproduce it as well using the master branch of Spectacle and Plasma 5.15... At the moment of the writing, I'm not exactly sure why I'm seeing this behavior...
This appears to be fixed. Significant changes happened between now and version 18.08, so it's hard to say when it might have been fixed, but it could have been version 23.04 or earlier.