From William Sit:

The fix is not as easy as it seems. It is not clear whether the "bug" should be fixed locally in combfunc.spad (a simple, temporary, but unsatisfactory fix is to modify ddprod and ddsum with paren(...)) with limited effect, or globally in OutputForm (outform.spad), where the bug was perhaps recognized:

    -- Todo:
    ...
    -- bug in product, paren blankSeparate []

The temperary fix is unsatisfactory because we should not SIMPLY add parenthesis to EVERY summation or product.  This is a general problem and requires an examination of the context to decide whether the parenthesis is necessary and that may be subjective. (I find controlling the output in Mathematica equally difficult, but generally, Mathematica did a much better job -- except in cases I
don't like :-). Since OutputForm is built bottom up (the responsibility of each domain via coerce), it may be difficult to make the decision even if it can be done objectively.