On one website where I also had these reappearing entries, I found them in the wp_options table in the key mainwp_child_actions_saved_data, which should be cleared after sending, but apparently isn’t. So I’ve deleted that key and tried to sync again. After that I deactivated a random plugin and activated it again. I’ve synced the child site on my dashboard and the entries were shown. I removed them and synced again and they didn’t reappear. So it’s solved for this site and the issue seems to be caused by the option that’s left in the database.
I’ve also tried to investigate the file in which this option is handled (mainwp-child/class/class-mainwp-child-actions.php), but couldn’t figure that out myself.