MINX not considering negative values

I think you should change your measure to this:

Running % Change = 
VAR __onlyOneFruitVisible = HASONEFILTER( Products[Name] )
VAR Latest_Week = 
    CALCULATE(
        MAX( 'Products'[Week] ),
        'Products'[Weight] > 0
    )
VAR First_Recorded_Week = 
    CALCULATE(
        MIN( 'Products'[Week] ),
        'Products'[Weight] > 0
    )
VAR Weight_FRW =
    CALCULATE(
        VALUES( 'Products'[Weight] ),
        'Products'[Week] = First_Recorded_Week
    )
VAR Weight_LW =
    CALCULATE(
        VALUES( 'Products'[Weight] ),
        'Products'[Week] = Latest_Week
    )
RETURN
    if( __onlyOneFruitVisible,
        DIVIDE( Weight_LW - Weight_FRW, Weight_FRW )
    )

Does this calculation make sense for many fruits at the same time? Probably not... Hence the IF guard clause.

First measure:

Greatest Decrease = 
var __decrease =
MINX(
    ALLSELECTED( Products[Name] ),
    [Running % Change]
)
var __decreaseFormatted = format( __decrease, "Percent" )
var __isDecrease = __decrease < 0
return
    "The "
    & if( __isDecrease, "greatest decrease", "lowest increase")
    & " in weight is " & __decreaseFormatted & "."

Second measure:

Fruit with Greatest Decrease = 
var __prodName =
    MAXX(
        TOPN(
            1,
            ALLSELECTED( Products[Name] ),
            [Running % Change],
            ASC
        ),
        Products[Name]
    )
var __decrease =
    MINX(
        ALLSELECTED( Products[Name] ),
        [Running % Change]
    )
var __isDecrease = __decrease < 0
var __result =
    "Fruit with the "
    & if( __isDecrease, "greatest decrease", "lowest increase")
    & " in weight is " & __prodName & "."
return
    __result

Here's the result. Just put the measures in two different cards. The measures react to the slicer on the right.

Best

Darek