Not able to find the error in the approach in Google Apps Script

Question

I import data from a google sheet into bigMatrix and then I apply filter on bigMatrix and filtered data is then assigned to a 2d array called matrix. When bigMatrix is inside the loop, everything is working perfectly. But when bigMatrix is outside the loop, then I was under the impression that I will be creating a new 2d array called matrix every time I apply filter on bigMatrix. bigMatrix is not changing anywhere in the code.

I am running a loop. I am not able to pin point the mistake.

Code 1:

var    bigMatrix = mainSheet.getRange(2,1,mainSheet.getLastRow()-1,mainSheet.getLastColumn())
                              .getValues();
  var           l = bigMatrix[0].length;   

  for(var w=0;w<years_list.length;w++){


  

  var purchase = [], sale = [], holdings = [], aio=[], matrix=[], matrixB=[], matrixS=[], filtered=[], writeSheet=[];
  var amc = [], amcArray = [], schemes = [], schemeArray = [], folios = [], folioArray = []
  
  var matrix = bigMatrix.filter(function(f){return f[4]<=(new Date(years_list[w],2,31))})
  
  matrix.map(f=>f[l]=0)

This is not giving me the desired result.

Code 2:

for(var w=0;w<years_list.length;w++){

  var    bigMatrix = mainSheet.getRange(2,1,mainSheet.getLastRow()-1,mainSheet.getLastColumn())
                              .getValues();
  var           l = bigMatrix[0].length;  
  

  var purchase = [], sale = [], holdings = [], aio=[], matrix=[], matrixB=[], matrixS=[], filtered=[], writeSheet=[];
  var amc = [], amcArray = [], schemes = [], schemeArray = [], folios = [], folioArray = []
  
  var matrix = bigMatrix.filter(function(f){return f[4]<=(new Date(years_list[w],2,31))})
  
  matrix.map(f=>f[l]=0)

This works fine.

As far as I know, filters create a new array. So why I have to read data from spreadsheet again and again.

Why can't I filter data from bigMatrix again and again without changing bigMatrix?