Sorry to be such a pest, but I found another unspecified behavior, this
time in string-split.

A problem arises if the delimiter has more than one character, and there
are overlapping matches for the delimiter.  For example, suppose the
delimiter is "aa" and the string is "faaab".  Does the delimiter match
the "aa" after "f", or the one before "b"?  It can't be both, and
choosing either seems arbitrary.

In any case, this should be addressed, either by explicitly saying it's
unspecified, or by specifying what should happen.