Expected behavior
Some people using normal e-mail clients might wonder why there is no meaningful subject in the email. For them it might be useful to get a link to the DeltaChat tool. Also as advertising for DeltaChat.
This could then be the standard behaviour which could be switched off in the settings. It makes only sense in unencrypted emails.
Actual behavior
Using other e-mail clients you see a standard subject (but not in DeltaChat).
Example Images
Subject: Message from name@…de — sent by DeltaChat
Off topic:
One other current issue is for example that in the answer from other email clients you see currently still this “Re: Message from name@…de” (if answering person did not change it) which is used from the DeltaChat sent message. As you do not see it in the reply in DeltaChat that might be no problem for the the DeltaChat user.