Currently Pattern.splitAsStream JavaDoc says [1]:

* <p> If the input sequence is mutable, it must remain constant during the
* execution of the terminal stream operation. Otherwise, the result of the
* terminal stream operation is undefined.

However in reality the sequence must remain constant from the stream
creation till the end of the terminal operation. Let's check:

public static void main(String[] args) {
   StringBuilder sb = new StringBuilder("a,b,c,d,e");
   Stream<String> stream = Pattern.compile(",").splitAsStream(sb);
   // Modify the CharSequence after stream creation
   sb.setLength(3);
   // During the terminal operation it remains constant
   stream.forEach(System.out::println);
}

The result is:
a
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 3
   at java.lang.AbstractStringBuilder.charAt(AbstractStringBuilder.java:210)
   at java.lang.StringBuilder.charAt(StringBuilder.java:76)
...