Well, I stand corrected.
Honestly, I am very surprised to hear that people are *paying* Google for GMail. I personally would advise against that, but given that you are, I would suggest getting their support involved in this discussion.
And I'm happy to hear that "it used to work" for some people, but I don't know how it ever did. GMail is not standard IMAP and we've heard in this discussion about different ways it fails with different IMAP clients. It's easy to point fingers at Apple's program (and I agree, it apparently still has some bugs, although I myself am not seeing any GMail-related ones after the update) but I still feel that this whole issue would have been avoided if GMail had simply supported standard IMAP from the start.
Not to get suspicious, but arguably GMail is deliberately different in order to steer customers to Google's official Mail reading app (and their web site). They prefer you to not use other IMAP programs because they want to control the ecosystem.
[As an example, I read that GMail began phasing out push notifications to Apple devices at the beginning of 2013. But if you use their official GMail App, you'll still get push notifications.]
Bark.