Nice. In case it helps, here are the places where that setting is set:
Targets
Occurrences of 'setPreference('ut'' in Project
Found occurrences in Project (4 usages found)
modules/UserPreferences (1 usage found)
UserPreference.php (1 usage found)
417 $this->setPreference('ut', $ut);
modules/Users (3 usages found)
SaveTimezone.php (1 usage found)
55 $current_user->setPreference('ut', 1);
User.php (2 usages found)
875 $this->setPreference('ut', '0', 0, 'global');
877 $this->setPreference('ut', '1', 0, 'global');
Maybe one of these is not using the correct logic anymore, for some reason…