After digging around, I see that in RH 2017 the mhfhost.js file has some new code that strips stopwords out of the search string. Since "and" and "not" are both stopwords, it is screwing up search. Workaround for us for now will be to remove those from the stopwords list.
One other thing I noticed after removing those terms from the stoplist: if you include "and not" in the phrase (i.e. you want to use boolean operators), you must also uncheck the “Include all words in search” box or else the search will not work.