"Do someone know why ?"
The documentation states "For character array inputs, strcat removes trailing ASCII white-space characters: space, tab, vertical tab, newline, carriage return, and form feed." It also suggests the workaround in the nest sentence: "For cell and string array inputs, strcat does not remove trailing white space."
strcat(Equation, {sprintf("\n")}, Equation2)
% ^ ^ scalar cell array
