EPPlus, .NET Core3 and setting value of Excel cell

Hi,

Trying to get & update an existing cell's value:

var q1 = worksheet.Cells["A11"].Value; // here I get a non-empty string
worksheet.Cells["A11"].Value = "new string";
var q2 = worksheet.Cells["A11"].Value; // returns EMPTY. WHY?????

I don't understand why my last string returns an empty value. How am I supposed to set cell's value in a correct way?