Seems like the YAML parser doesnt recognize tagged scalar nodes really well;

a: !foo bar
b: !foo 'bar'
c: !foo
    bar
d: !foo [1, 2, 3]

array:4 [▼
  "a" => "!foo bar"
  "b" => "!foo 'bar'"
  "c" => TaggedValue {#254 ▶}
  "d" => TaggedValue {#221 ▶}
]

Considering DI; both !iterator foo and !iterator [foo] produce a valid container, but a different value.