Hi @Bernardamus,
Thanks for getting back to me.
Apologies, I didn’t know that your end goal was to build a report, I though it was a list.
Althought your workaround works, please note that withing Reports, you can build a cross-object (contacts/activities) custom report, then go to filters > activities > and find the same “Email subject = is known”
a GIF video representing this below, let me know if you have any doubts:
