Look at the following document:

  \Umathlimitbelowbgap\displaystyle0pt
  \Umathlimitbelowvgap\displaystyle0pt
  $$
    \sum_a^b
  $$
  \bye

Here the `a` subscript gets lost. In mlist.c, make_op, line 3324 the
subscript (stored in variable `z`) is never coupled with the previous
node if `shift_down` is smaller or equal to 0.