Aqua Mail only ever was able to look up contacts in the global directory for Exchange accounts (not Gmail IMAP accounts).
OK.
I assume it's the same in "other third party apps", but can't know the specifics of those.
Not quite true. Applications like Nine and Boxer are successfully looking up email addresses in the our Global Address Directory and that is held in Gamil. I checked most of the email apps in the App store and I'd say about half of them do manage to look up the addresses and about half do not (like AquaMail).
I can tell you with certainty that Aqua Mail never did anything "special" to look up addresses in a global directory for any account type other than Exchange.
I suspect you are correct. I suspect that up to a few weeks ago, those contacts in our Global Address Directory were all synched into the Contacts on my phone and that is how AquaMail found them. Mind you, I may be completely wrong about this. It's just I do seem to recall hiding all those work contacts at one point in my phone.
It's a real shame AquaMail doesn't look up the contacts. After checking almost every email client in the app store, I can tell you that AquaMail is by far the best. But without this feature, I will need to abandon it.
Thanks anyway.