problem (bug?) with fill prefix in latex mode

[Martin Rubey]

I have the following text in a latex mode buffer

      ++ maxMixedDegree(d) specifies the maximum q-degree of the
      ++ coefficient polynomials in a recurrence with polynomial
      ++ coefficients, in the case of mixed shifts.  Although slightly
      ++ inconsistent, maxMixedDegree(0) specifies that no mixed shifts
      ++ are allowed. This option is expressed in the form
      ++ \spad{maxMixedDegree == d}.


setting fill-prefix to "      ++ " and pressing M-q turns it into

      ++ maxMixedDegree(d) specifies the maximum q-degree of the ++
      coefficient polynomials in a recurrence with polynomial ++
      coefficients, in the case of mixed shifts.  Although slightly
      ++ inconsistent, maxMixedDegree(0) specifies that no mixed
      shifts ++ are allowed. This option is expressed in the form ++
      \spad{maxMixedDegree == d}.

Help for fixing that would be greatly appreciated!

Martin