While working with Emails when legacy form rendering is enabled, I found one bug in Dynamics 365 which blanks From field and locks Regarding field as soon as email changes is saved on Received emails. This bug can be reproduced using following steps:
Let say we have one queue configured, our sales team sent one email to this queue, you can see in following email, under From field sales team address and Regarding field is blank.

Now I have created one contact with our sales team email address and I want to set Regarding to that contact. So I look for this email address and after setting regarding field as soon as, I save email form it locks Regarding field and makes From field blank like following:

Resolution: As a workaround we need to reload email form to bring Form field value back.

Hope it will help someone !!