Behavior in LibreOffice / Excel:
The result of function call FLOOR.XCL(-123;10) in LibreOffice 6.1.2.1 is -130.
The result in Excel Office 365 is also -130.
The result in POI 4.0.0 via cell.getNumericCellValue() is -130.0.

Problem:
The result of new DataFormatter().formatCellValue(cell, formulaEvaluator) is #NUM!.

Expected result:
I suppose that the result in POI also should be -130.

Idea for improvement:
Remove the condition (n<0 && s>0) in the if-statement in org.apache.poi.ss.formula.functions.MathX.floor(double, double) 

And I suggest that the condition (s==0 && n!=0) in org.apache.poi.ss.formula.functions.MathX.floor(double, double) could be removed also. Excel returns 0 for floor(0;10) and LibreOffice also returns 0.

I guess that org.apache.poi.ss.formula.functions.MathX.ceiling(double, double) has already been changed this way, because the docs tell about negative n, but the if-statement does not check that.