It’s not exactly a bug. Like other “zero” bots, KataGo basically uses Tromp-Taylor rules (https://senseis.xmp.net/?TrompTaylorRules), which are the simplest possible form of rules to implement. Under these rules, you get only score for the space you surround that has no opponent stones in it. So if there are “dead” stones, you have to actually prove them dead by capturing them.
If you end the game without doing so, then technically under Tromp-Taylor, that area is not your territory yet, and passing would indeed be a fatal mistake (much like it might be a mistake under human rules to pass when your territory has an unfinished border, regardless of whether you would be winning if the border was finished). So it’s a little less of a “bug” and more like “mismatch between computer rules vs human rules, computer is analyzing correctly under one particular ruleset but that ruleset is the wrong one for human analysis”.
@anoek For now, you can fix this just by skipping analysis on any move that follows an opponent’s pass. You can resume analysis on any moves after that pass if they are not pass moves, because on any moves that are not immediately following a pass, there should be no danger of the game ending.
I’ll add a hack in the next release of KataGo where you can configure it to assume that passes cannot end the game for the purpose of analysis, which should address the rules mismatch here.