I have my isearch failure face set like so:
(custom-set-faces ... '(isearch-fail ((t (:background "#660c00")))) ... )
It works for the most part, highlighting text which doesn't match in that color. However, whenever I write failing whitespace, i.e. a space or a tab which has no match in the buffer, the face that is used to highlight it is a brighter shade of red which is not the same color as the rest of the unmatched text. Furthermore, this color of whitespace is replaced by my custom isearch failure color when non-whitespace characters are written after it.
I suspect that I can solve this issue by setting some face somewhere, but I'm not sure which face is the culprit here and I've looked and looked for it to no avail.
FYI, I'm using GNU Emacs 126.96.36.199 on OS X 10.11.6.