During the final visual check of tax forms, I see that line 5a of 1040 is blank yet line 5b has a correct entry. Here, 5b is supposed to be the taxable portion of 5a.

I have posted this myself and I see the same thing that you do.  It will only post a value to 5a if it is different from 5b.

With 5b posted it is all you need to complete your return.