version: 3.0.1
happens without third-party customizations: yes
regression from fish 2.7.1: does not apply

echo -n -e "a\x00b" | string split0 correctly splits between a and b, but exits with status code 1 despite having performed a split. This is contrary to the statement in the documentation that if a split was performed, string split[0] returns with status code 0.
Compare echo -n -e "a b" | string split " ", which correctly exits with 0.