With that comment about the vocalist that got confused with the background "spitting out" all sorts of confusing "backing"...I recall how another program, that I truely love
the iRealBook, that offers you "simplified", however disconcerting trio backing...to even those who have sung jazz for years, mostly utilizing extended or "drop chords"...

Well, altho my ear and memory are pretty good at picking up on a tune immediately, even with an extra sexy (dissonant??) intro by the accompanyist (even me)...
when I go into a iRealBook tune (an overwhelming number of the "standards" in the electronic fake book are uptempo) and so...

With just a two-bar countdown before "this backup trio" takes off into the number, more than once it's taken me a "run through" or at least a few bars into the tune before I
pick up on the actual "pro" chords that suddenly appear "at tempo"!
It could be a tune I'm happy to have done many times before...but some of these melody lines can be hidden pretty good by those chords.

It isn't surprising to me that many vocalist can get tripped up by the jazzy chord stuff!

Some time, even slowing the tempo of the "cooked-up" changes, doesn't help all that much.

That's why I have such great respect for many of females, Like Jane Monheit, Shirly Horn, and scores of other jazz singers, these gals have a natural (or studied.. even), ear... for "taking chords in" with what seems as an uncanny "ease"!!