Using the following documents:
Running aggregation operator $sqrt as below:
Note that the output of the $sqrt is the same as the output of $exp.
Looking at the code, the method Decimal128::squareRoot is exactly the same as Decimal128::exponential :
A workaround is to convert the decimal to double first i.e. $toDouble, to bypass the squareRoot method call in expression: https://github.com/mongodb/mongo/blob/r4.1.6/src/mongo/db/pipeline/expression.cpp#L4091