screenshot keyboard shortcuts depend on release order

[milesf]

According to this guide, the following keyboard shortcut should capture the current window to clipboard:
Ctrl + Alt + Prt Scrn
https://help.ubuntu.com/stable/ubuntu-help/screen-shot-record.html

Sometimes this sequence will save to the ~/Pictures folder instead.
It depends on the release sequence of the keys. When Ctrl and Alt are released at together, the outcome is seemingly random.

If Ctrl is released before Alt, then the screenshot will unexpectedly bypass the clipboard and be saved to ~/Pictures.

Key sequence to reproduce unexpected behavior:
Press and hold Ctrl
Press and hold Alt
Press and release Prt Scrn
Release Ctrl
Release Alt

It's also strange that the screenshot is actually captured when either Ctrl or Alt is released, and not when Prt Scrn is pressed or released.
For example:
Press and hold Ctrl
Press and hold Alt
Press and Prt Scrn <- Screenshot is not captured here
Release and Prt Scrn <- Neither is screenshot captured here
Wait 10 seconds
Release Alt <- This this is when screenshot is captured
Release Ctrl

Ubuntu 18.04