Google Apps Script getPlainBody() from GmailMessage class regex not working

this is my first question on stackoverflow so plz let me know how I can improve readability for others.

Am trying to use regex on the string I obtained from getPlainBody() in GmailMessage Class but somehow it doesn't work when I try to do it directly on the string returned by getPlainBody() but works well when I manually add \n characters.

Code that works:

function RegularExp() {
  
  //manually entered \n characters into string that I copied and pasted from getPlainBody()

  var string = "Personal Message\nraw material: oak wood 100kg\nTRACKING NUMBER 7777777777\n<somehyperlink\nFROM SomeBrand"; 

  //my goal is to get: raw material: oak wood 100kg

  var regExp = new RegExp("(.*?)\n(?=TRACKING NUMBER)","g"); 

  var PersonalMessage = regExp.exec(string)[1];
  Logger.log(PersonalMessage); //works perfectly fine

}

Code that doesn't work:

for (var j in messages){
  var message = messages[j];
  var plainText = message.getPlainBody(); //getting plainbody of fedex mail of interest

  //trying to extract the personal message 
  var regExp = new RegExp("(.*?)\n(?=TRACKING NUMBER)","g");  
  var PersonalMessage = regExp.exec(plainText)[1];
  Logger.log(PersonalMessage); //won't show anything
}

My question is why does it work when I manually enter \n but not when I use the string that was returned from getPlainBody()? I'm using the exact same regex pattern and can't see why.

Below are the links I used to try to solve my problem (or I might just be dumb not being able to apply the solution to this issue)

Newline in gmail app script getplainbody function

Google Apps Script: getPlainBody() weird behavior

Regex - google apps script

Thanks