Why are email replies sent from iPhone not threaded in Gmail? I noticed that when replying to emails from my iPhone, they are no longer being grouped as a conversation in Gmail.

If I receive an email and reply via my iPhone email app, it will show up in my send folder on my desktop.  But not grouped in the conversation thread like it should.  

Until recently, no matter where I replied from, the reply always ended up as a reply in the same conversation.

When I reply from my desktop, it does show properly as part of the conversation. Why?
Apparently, there's something wrong with the iPhone 6, its configuration is not fully compatible with Exchange settings.