It is a recent regression in Konsole (introduced somewhen on >=2018-05-09).

Steps to reproduce:
1. Find some longer word or write it (e.g. "aaaaaaaaaa")
2. Double-click this word to select it, keep mouse button pressed and move mouse cursor to the right

This results in additonally selecting text (often whitespace) located right to mouse cursor.

The longer the word, the easier to notice this problem.