MailPoet Emails Subject modified automatically

Using MailPoet Version 3.0.0-beta.20
The “Subject” of my Emails is often changed when I start editing the Email.
The link of the edit button is
https://example.com/TestPlugin/wp-admin/admin.php?page=mailpoet-newsletter-editor&id=4
After clicking the link, the Subject is often changed. When it is changed, it is always changed to the same value (“bolero20”) and the change is saved (even if I do not click Save of Next, but Emails instead).
The “Last modified on” field in the overview table is also changed https://example.com/TestPlugin/wp-admin/admin.php?page=mailpoet-newsletters#/notification
This happens with Newsletters, Welcome Emails and Post Notifications.