Running WordPress with WP Matomo under PHP 8 I got a couple of warnings:

• Warning: Undefined array key "nb_actions" in WP_Piwik\Widget\Post::show at line 44
• Warning: Undefined array key "bounce_count" in WP_Piwik\Widget\Post::show at line 45
• Warning: Undefined array key "sum_visit_length" in WP_Piwik\Widget\Post::show at line 46

I think an easy fix would be checking if they exist and otherwise use 0.