Report: https://www.phpbb.com/community/viewtopic.php?p=16082743#p16082743

Diff engine declares $final1 and $final2 properties of class diff3_op as protected:

https://github.com/phpbb/phpbb/blob/a063b534eb7441f30251e6e470bd255ccd74eb2a/phpBB/includes/diff/diff.php#L771

https://github.com/phpbb/phpbb/blob/a063b534eb7441f30251e6e470bd255ccd74eb2a/phpBB/includes/diff/diff.php#L776

but refers them dynamically from outside:

https://github.com/phpbb/phpbb/blob/a063b534eb7441f30251e6e470bd255ccd74eb2a/phpBB/includes/diff/diff.php#L570

Hence the error in advanced update package:

PHP Fatal error: Uncaught Error: Cannot access protected property diff3_op::$final1 in [ROOT]/install/update/new/includes/diff/diff.php:570