I'm not totally familiar with how AcyMailing subscribes new users. With JFBConnect, when we create a new user, we go through this flow:
* Create a new instance of JUser
* Populate it with the user's info (name, email, etc)
* Call the 'save' function
The save function performs all of the normal Joomla checks for a user and triggers the following plugin events:
* onUserBeforeSave
* onUserAfterSave
Most extensions that check for new users to 'do' something use those events to check if the user is new. Then, if so, they can add them to a newsletter or whatever else is needed.
If AcyMailing is not using those events and doing it some other way, that may be the problem. It's something you may want to verify with the Acy team first to check if there's anything missing in the flow. It's something we can add to our to-do list, but I can't guarantee any timeframes for when we'd have an answer.
I hope that helps explain,
Alex