The basic problem is that you are storing scalar strings in a cell array. This should be avoided: "If you create variables that have the string data type, store them in string arrays, not cell arrays"
"Avoid using cell arrays of strings. When you use cell arrays, you give up the performance advantages that come from using string arrays. And in fact, most functions do not accept cell arrays of strings as input arguments, options, or values of name-value pairs."
Lets simply convert those strings into character vectors using the (for historical-reasons rather badly named) CELLSTR function (which actually converts into a cell array of character vectors):
C = load('matlab_cell.mat').newList;
C(:,1) = cellstr(C(:,1));
C = sortrows(C,1)
