After using CTRL + F12 twice (show the Desktop and hide it again), the previously focused window is not focused anymore. In fact, no window at all is focused. I think the best default would be to re-focus the previously focused window.
This is a duplicate of https://bugs.kde.org/show_bug.cgi?id=375993. *** This bug has been marked as a duplicate of bug 375993 ***
For the record: I created the bug report first ;) But thank you for your triaging time.
You did, and in general it's a shame to forward-dupe like that. But the bug with more attention is the one more likely to get fixed!