Shared Channels: @Channel mentions fail to trigger activity notifications for team-shared members

So up until last week we had the same behaviour

"When someone uses a channel mention (e.g., @ChannelName) in this shared channel, the users who have access via the team-sharing mechanism do not receive any activity feed notifications."

But:

Now I have created two new teams and shared a channel from 1 to 2 and this thing just works? Where is the documentation for this?