Upon further investigation, I answered my own question as follows: in the problem in question, we have

Context("Vector")->variables->are(t=>'Real');

Context()->reduction->set('(-x)-y'=>0);

and the further down in the same problem:

Context("ImplicitPlane");

So any updates done to the "Vector" context (including the reduction->set in the original problem text) apparently have no effect for the later typesetting of the implicit planes. However, if I put

Context()->noreduce("(-x)-y","(-x)+y");

*after* the Context("ImplicitPlane"), then the undesirable formatting can indeed be turned off. The reason my earlier efforts had no effect is that I put the "noreduce" command after Context("Vector") near the top of the problem.

For this problem, I suggest adding

Context("ImplicitPlane");

Context()->noreduce("(-x)-y","(-x)+y");

Thanks, -- Peter