All these files should be sourced, not executed; and Debian's lintian complains:
W: zsh-syntax-highlighting: script-not-executable usr/share/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh
Fixeszsh-users/zsh-syntax-highlighting#188 in the case that both the
opening '((' and closing '))' have been typed, The case that only the
opening '((' have been typed is also fixed, but requires a zsh development
build (zsh-5.1.1-52-g4bed2cf or newer); see comments within.