Symfony version(s) affected: 5.x-dev

Description
Symfony Yaml parser incorrectly parses some block sequences.
In particular, if using this test data, the output produced is

[
  "- s1_i1 - s1_i2",
  "s2"
]

which is not the expected output as per the test.

How to reproduce

<?php
require_once '../vendor/autoload.php';

$yaml = <<<Y
- - s1_i1
  - s1_i2
- s2
Y;


echo(json_encode(\Symfony\Component\Yaml\Yaml::parse($yaml)));

Additional context
Related issues:

• Project.yaml not populating dropdown field options craftcms/cms#5777
• Stripping meaningful whitespace from YAML files prettier/prettier#10506