IMO there should be no sub-sinking-sub scenario in the SH5 at all.
As far as I know there was only ONE confirmed sinking sub by another sub in the entire WW2.
The scenario in Mediterranean requiring you to sink THREE british submarines to accomplish it is (so far) the most derailed and retarded idea the devs came up in SH5.
In my years of playing SH3GWX I managed to sing an enemy sub ONCE...!
On the other hand I agree, you should get credited or it.