nansum does not do what you are expecting it to do. Also, using nansum() is not recommended as you can see from its documentation page. Use sum instead -
Here's a workaround -
%
soma_total = nan(3, 4);
data1 = [nan nan 1 2; 1 2 nan nan; 1 2 nan nan];
data2 = [5 5 nan nan; 2 1 1 nan; nan nan 1 1];
%Dimension to operate on
dim = 3;
%defining the array
arr = cat(dim, soma_total, data1, data2);
%Finding indices for all elements which are NaN corresponding to the "dim" dimension
idx = all(isnan(arr),dim)
%If you want to use nansum instead, use this command
%result = nansum(arr,dim)
result = sum(arr,dim,'omitnan')
%Converting the respective values back to NaN
result(idx) = NaN
