The docstring for join says:

join([io::IO,] iterator [, delim [, last]])
Each item of iterator is converted to a string via print(io::IOBuffer, x).
If io is given, the result is written to io rather than returned as a String.

This is now not true because an AnnotatedString turns into a String when printed and extracted from an IOBuffer and the docstring says the result should be a String which is no longer true:

julia> s1 = Base.AnnotatedString("abc def", [(1:3, :face=>:yellow)])
"abc def"

julia> join(s1) |> typeof
Base.AnnotatedString{String}

AFAIU, the docstring should now be something like:

If any element of iterator is an AnnotatedString the result is also an AnnotatedString, otherwise the result is a String