When someone sets a target ACOS at the campaign level -> all the keywords in there inherit the Target ACOS.
If a user then goes in and ADDS ANOTHER keyword to that ad group, they should set a target ACOS for that keyword.
We're planning to introduce a prompt when users add keywords or graduate items. By default, we haven't implemented this because our users have the flexibility to set individual target ACOS and minimum/maximum bids for each keyword. However, to simplify the process for our users who prefer not to customize, we'll soon be prompting them with the option to either use the campaign settings or customize it right away.