That is a feature that has been required to implement invokeOnComponent() properly. Client identifiers have changed that way since 3.2.0. You should better use getClientId() function to obtain client ids.
I looked through JSF specification and didn't find anything that prohibits usage of double colons in identifiers.
No problem, I was concerned that the double colon was just a bug and that in a future release it would go back to a single colon.
The other concern is that "::" implies an unnamed naming container, but if it works it is not a problem.