As per my understanding the "assignment block" copies the input vector and update it in each step due to which these spikes occur. Use "MATLAB function block" instead of assignment block to assign the vector and remove the spikes.
Refer to the following code of the MATLAB function block:
function y = fcn(u, data, idx)
persistent temp
if isempty(temp)
temp = u;
end
temp(idx) = data;
y = temp;
