In version 5.3.1 the invert selection tool always selects a portion outside the canvas, this didnt happen on the last version and I believe its unintended.
Just to be clear, this is a new canvas, I only made that square for this example so its not a stray pixel.
Hello @r_d and welcome to the forum 
This has been reported before, here: Krita 5.3 Selection Update Issue
A formal bug report has been made and Confirmed: https://bugs.kde.org/show_bug.cgi?id=518235
Now we must wait patiently for the developers to investigate it and fix it.