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Okay, just to add to this, I just tested this specifically using my own account — I created a new "personality" for my Gmail address in the classic interface, and then tried sending messages from there using both the classic and the new interfaces.
Again, I can't speak to what eumx might specifically be doing, as I don't have an account there, but based on the examination of the headers, I can't see any indication of my original address being disclosed, so I don't even see anywhere eumx could possibly be getting it from.
In fact, even if you're NOT using Gmail's SMTP server for your Gmail personality/identity, while the message will fail certain checks, there's no way for them to determine your FastMail address from that, merely that the message came from a server not authorized to send mail for Gmail.com (and the server name in that case wouldn't even be FastMail, but rather "messagingengine.com").
There's also still nothing I can see that would explain why it would work in classic but not in the new interface, other than some difference in how Gmail personalities are handled. I'd still recommend trying to re-create the Gmail personality as an "identity" in the new UI and see if that helps, as the new UI takes specific steps to authenticate and configure a Gmail account "properly" rather than letting you tweak settings manually.
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