When sending a download invitation or notification, the email subject is always Mailable.

When sending a test email over settings -> mail the subject is correct.

Friendly regards, Tim

I have searched the entire plugin for the words "Mailable" and "subject". The only place "subject" appear is in the mail template files.

Have you made your own templates?

I guess it is related to https://github.com/octobercms/october/issues/3384