Force Case Email Quick Actions to use predefined Subject values when invoked from Reply button in Email feed

Question

I have 3 Send Email Quick Actions on Case, no Default Template selected, with predefined values for Subject. When invoking these Actions from the Reply or Reply All buttons at the bottom of an email (see screenshot below), the email uses the subject from the email being replied to, not the predefined subject. The Don’t Apply Template Subject checkbox on the Action doesn't affect this behavior. The use case requires using these buttons under the messages to preserve the addressees & the message content, rather than invoking them "fresh" from the top of the feed (where the predefined subject works fine). Is there a way to make the Actions use the Predefined Value for Subject when invoked this way?

Answer 1

I got a response elsewhere from someone who has already pursued this issue via Salesforce support, and was ultimately pointed to the 4th bullet under Predefined Values in this Help article:

The predefined field values for the To Recipients, CC Recipients, and BCC Recipients fields are filled only for the initial email. The fields aren’t pre-filled for reply emails or forwarded emails.

Even though this doesn’t specify that this behavior applies to Subject as well, that’s what Support told this other customer is the case.

I'm currently pursuing a solution through Apex's QuickActionDefaultsHandler interface.