For demonstration, here’s a video I recorded on 5.2.x (git 939408c), just repeatedly using the line tool for a few minutes; brush is “b) Basic-2 Opacity”. Before that I was using “b) Basic-1”, but nothing was happening, so I switched brushes. Could be it only happens with certain brushes, could be the act of switching brushes somehow triggers it, could be I was just (un)lucky. After a while it just randomly happens, trying to undo it causes a safe assert in libs/image/tiles3/kis_memento_manager.cc, line 275, and redoing in line 332.
(For fun I turned on all the logging options. The Unknown transform parameter : "" being logged is from kis_dynamic_sensor::id2sensor(). The brush is trying to use some sort of incorrect sensor or something, but the message is very unhelpfully worded.)