Yes, without the quotes, the IPv6 address gets interpreted a YAML mapping/dict because of the colon(s).
Perhaps the trap is the complacency that YAML induces by not requiring quotes around keys/values, and so text risks being interpreted in unexpected ways. The infamous Norway Problem has the same root cause.
Even in the first position? That makes it no longer be the claimed "superset of JSON", since (AFAIK) {"key":"value"} (with no whitespace) is valid JSON.
For delimited collections, so within {} and [], if the key is quoted, then you don't need the whitespace. So your example parses as expected as does `{"key":value}`, but `{key:value}` turns into `{"key:value": null}`.
Perhaps the trap is the complacency that YAML induces by not requiring quotes around keys/values, and so text risks being interpreted in unexpected ways. The infamous Norway Problem has the same root cause.