An additive change breaks a strict validator

Adding a field is safe by convention and unsafe in practice, because a client generated from a schema with additionalProperties: false rejects it.

{
  "type": "object",
  "properties": { "id": {}, "total": {} },
  "additionalProperties": false
}

// so the compatibility rules go in the documentation, as
// a contract rather than an assumption:
//   clients MUST ignore unknown fields
//   clients MUST NOT depend on field ordering
//   we MAY add fields and enum values in a minor version

Stating what may change is more useful than promising nothing will, because it tells a client author which assumptions are theirs to fix. Adding an enum value is the case that catches even careful clients — a switch over known values gains an unhandled case — and whether that is your fault depends entirely on whether you said it could happen.