I just tested this here using VC for VS2015 2.3 update 2 and both reference modified and subscripted items display correctly when I hover over them in debug and the option for COBOL Debug Tooltip Style is set to Show COBOL occurs or referenced modified value.
In 2.3 update 2 this option has been moved to the context menu which is displayed when you right-click while debugging is in effect.
What product version are you using?