I think you will agree it's not a detail.  The reason we want to
highlight matching parts in flex is the same reason grep and every
search tool and search engine I know decides to highlight matching
parts: to call attention to the part that matched.

Of course, fixing that crippled default state of 'flex' is a couple of
customizations away (Put the common face to 'bold' and the
first-difference to nothing).  But, IMHO, it would be a shame if we were
to release Emacs 27 with this familar matching method and no good
default faces to go with it.