Have an Item Modifer that has a "set_count" parameter of any number (i.e. 3) with "add" parameter set to "true". Which should make the change relative to the current count.

Have that same item in an Inventory slot with any count (i.e. 5)

Use the item command with this modifier to modify the Inventory Slot

The count is expected to be 8 because the modifier should have made the change relative to the current count, however the count is 4.

[    
     {      
          "function": "minecraft:set_count",       
          "count": 3,      
          "add": true    
     }
]  

If this is intentional behavior the "add" parameter is unnecessary because in that case you might as well just have the "count" set to 4 initially.