SonarCloud reports:
  Cast one of the operands of this subtraction operation to a "long".

Here:
        Spliterator<SplittableGenerator> makeSplitsSpliterator(long index, long fence, SplittableGenerator source) {
            ...
            long multiplier = (1 << SALT_SHIFT) - 1; // <---- here

The shift is integer, and the cast to long is too late. SALT_SHIFT is currently 4, so this is not the problem. But it would become a problem if SALT_SHIFT ever becomes 32 or larger. The shift operand should be 1L for safety. Observe:

jshell> Long.toHexString((1 << 31) - 1)
$2 ==> "7fffffff"

jshell> Long.toHexString((1 << 32) - 1)
$3 ==> "0"

jshell> Long.toHexString((1L << 32) - 1)
$4 ==> "ffffffff"