Drag n Drop decoys previously working fine, but no longer. Maybe linked to a recent SL 360 release?

Greetings!

A drag-n-drop interaction to select a correct ingredient out of a choice of six was working as it should up until some months ago. The course has essentially been in wait mode since then as translations and a sister course are completed. Now, however, one aspect of the drag-n-drop has been discovered to no longer be working as it was, without having changed anything. 

The learner can drag as many ingredients of the six as they like to the drop target, and then submit. Only one of these ingredients is however correct, but it is a common misconception that there are more than the one ingredient (thus the exercise.) So five of the ingredients are decoys, with 'none' selected as drop targets.  

The desired behaviour, previously achieved, is for the 'Try Again' feedback to launch for any incorrect drops, even if the correct ingredient is among other incorrect ingredients (in which case we trigger a 'partially correct' message as a hint.) Learners get five tries.

Now however, having not changed anything in the course page, dropping multiple ingredients where the correct ingredient is included does not launch the 'Try Again' feedback layer. Pressing Submit gets no reaction. 

If single or multiple incorrect ingredients are dropped without the correct ingredient, then the functionality works (i.e., Try Again.) If the single correct ingredient is dropped, the Correct layer is evoked as it should. 

I can even see an earlier test version published on Articulate 360 showing the desired behaviour. But when I now load that source file from these earlier versions into the latest SL 360 software, the problem described above occurs. 

Which could suggest that something fixed or enhanced in a recent release has had an inadvertent knock on effect. Maybe?

Very keen to get this resolved, as launch is now approaching.  Thanks!