The documentation for solve does say,
Solve this system of equations and assign the solutions to variables b and a. To ensure the correct order of the returned solutions, specify the variables explicitly. The order in which you specify the variables defines the order in which the solver returns the solutions.
Keep in mind, though, that the "order" for that purpose is relevant to using the
[var1, var2, ....] = solve()
form of solve. When you assign to a single variable on the left hand side, the output will be a structure, and struct2cell() is going to give you whatever order the fieldnames happened to be in for the struct. If that is important to you then,
numvar = length(X);
sols_cell = cell(numvar, 1);
for K = 1 : numvar
sols_cell{K} = Solutions.(X{K});
end
