Thank you, Patrick.
I have edited as you have showed, but still, whenever I drill down and drill up, my custom formatting gets reset. What I ideally want is that no matter how granular I go on my view, custom formatting applied: MMM 'YY would be kept as it is defined.
From my experimenting I came to conclusion that this is not possible in tableau, unless I use 3 or 4 different views and link them with a parameter.
But I believe that even then an end-user can just click on the plus and minus signs and replicate the same issue. (Unless I cover plus and minus signs)
With kind regards,