This came up while investigating BF-35904. The minimized query looks like this

{
    "$group": {
        "_id": null,
        "num": {
            "$stdDevSamp": {
                "$sum": [
                    NumberLong("20439413136"),
                    "$measurement1"
                ]
            }
        }
    }
}

The explain output for optimizations turned has the operands swapped like so

"$group" : {
    "_id" : {
        "$const" : null
    },
    "num" : {
        "$stdDevSamp" : {
            "$sum" : [
                "$measurement1",
                {
                    "$const" : NumberLong("20439413136")
                }
            ]
        }
    }
}, 

while with optimizations off the operands remain in the same order

"$group" : {
    "_id" : {
        "$const" : null
    },
    "num" : {
        "$stdDevSamp" : {
            "$sum" : [
                {
                    "$const" : NumberLong("20439413136")
                },
                "$measurement1"
            ]
        }
    }
}, 

this change comes from ExpressionNary::optimize() as it will hold the ExpressionConstant operand in a separate array and push it back to the operand array at the end. This simple reordering causes the _addend inside DoubleDoubleSummation to differ in the 5th decimal place, which then propagates to have the result of the sum (which is pulled into AccumulatorStdDev::processInternal()) differ in the ~16th significant figure, and then _m2 will differ in the ~10th significant figure.

I also confirmed that the results don't differ if the input query has the field path before the constant, like this

{
    "$group": {
        "_id": null,
        "num": {
            "$stdDevSamp": {
                "$sum": [
                    "$measurement1",
                    NumberLong("20439413136"),
                ]
            }
        }
    }
}, 

This failure was reproduced deterministically in a VWS running ubuntu on ARM, on a regular collection with no indexes. This might also happen with other accumulators but I did not test that.

This could be considered not a bug but we might want to add a rule to the fuzzer to accept this as a close enough result, probably inside assertAggFuzzerResultDivergenceIsAcceptable().