See this comment from formats helper:

• Note that type hints for each $value argument are avoided to allow for these callbacks to be executed when columns are
• aggregated using one of the "Group concatenation" methods, where the value is typically stringified

This is easy to miss when implementing column callback methods, and causes PHP to throw TypeErrors. We should fix this, to ensure the original type of the column is preserved when passing values to it's callbacks during group concatenation aggregation

For context, see this change from MDL-72663 where type hinting was removed from existing callbacks (we should revert that part if possible)

This would be nice to correct now, as we are starting to add new report sources for 4.1 (MDL-73987)