On message type "conversation.item.input_audio_transcription.completed" returns prompt

Hello. any idea, why sometimes, occasionally, on type: “transcription_session” or “session”, when you get “conversation.item.input_audio_transcription.completed” event, sometime this event returns prompt described in configs? configs looks like:

{
  object: "realtime.transcription_session",
  id: string,
  input_audio_format: string,
  input_audio_transcription: [{
    model: string,
    prompt: string,
    language: string
  }],
  turn_detection: {
    type: "server_vad",
    threshold: float,
    prefix_padding_ms: integer,
    silence_duration_ms: integer,
  } | null,
  input_audio_noise_reduction: {
    type: "near_field" | "far_field"
  },
  include: list[string] | null
}

lets say prompt is “you are listening to call related to coding”, and when “conversation.item.input_audio_transcription.completed” event comes, it would return this exactly prompt. any ideas?