I think the problem I'm having is related to Settings > Manage Identities > Email Address > Edit > Copies and Folders... where I have a 'sub' email address save a copy of sent emails to a specific folder. It seems TB wants to change the icon of the folder it's saving to from a folder icon to a sent icon. The behavior I'm trying to create... Email arrives to purpose-specific email address, drag it into purpose-specific folder, reply to it, have sent reply automatically store in same folder. So that the exchange is easy to follow.

Okay, I think I've got it now. If you have 'sub' email Identities save copies of Sent emails from that address to a purpose-specific folder, it automatically changes the icon of the folder to the 'sent' icon. Regardless of what other emails (i.e. received purpose-specific sub email identity correspondence) you have in that folder. i.e. You can't have a folder that receives copies of sent emails from Identity email addresses retain a Folder icon, they automatically change to a Sent icon. I think this might be a bug.