ffmpeg – MP4 file starts from a non-key frame

I have used the following ffprobe command to analyse a .mp4 file.

ffprobe -i <input> -show_frames - select_streams v:0 -print_format flat &> save_to_file.text

It produces the following output.

    ffprobe version 5.1.3 Copyright (c) 2007-2022 the FFmpeg developers
      built with gcc 13 (GCC)
      configuration: --prefix=/home/thanuja/ffmpeg_build --pkg-config-flags=--static --extra-cflags=-I/home/thanuja/ffmpeg_build/include --extra-ldflags=-L/home/thanuja/ffmpeg_build/lib --extra-libs=-lpthread --extra-libs=-lm --bindir=/home/thanuja/bin --enable-gpl --enable-libfdk_aac --enable-libfreetype --enable-libmp3lame --enable-libopus --enable-libvpx --enable-libx264 --enable-libx265 --enable-nonfree --enable-openssl --enable-demuxer=spdif --enable-decoder=dolby_e --enable-decoder=ac3 --enable-decoder=eac3 --enable-indev=alsa --enable-outdev=alsa --enable-shared
      libavutil      57. 28.100 / 57. 28.100
      libavcodec     59. 37.100 / 59. 37.100
      libavformat    59. 27.100 / 59. 27.100
      libavdevice    59.  7.100 / 59.  7.100
      libavfilter     8. 44.100 /  8. 44.100
      libswscale      6.  7.100 /  6.  7.100
      libswresample   4.  7.100 /  4.  7.100
      libpostproc    56.  6.100 / 56.  6.100
    Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'FTV267StoS.mp4':
      Metadata:
        major_brand     : mp42
        minor_version   : 1
        compatible_brands: isommp41mp42
        creation_time   : 2023-04-02T23:52:12.000000Z
      Duration: 00:04:47.84, start: 0.000000, bitrate: 7374 kb/s
      Stream #0:0[0x1](und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709/unknown/bt709, progressive), 1920x1080 [SAR 1:1 DAR 16:9], 7198 kb/s, 59.94 fps, 59.94 tbr, 90k tbn (default)
        Metadata:
          creation_time   : 2023-04-02T23:52:12.000000Z
          handler_name    : Core Media Video
          vendor_id       : [0][0][0][0]
      Stream #0:1[0x2](eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 125 kb/s (default)
        Metadata:
          creation_time   : 2023-04-02T23:52:12.000000Z
          handler_name    : Core Media Audio
          vendor_id       : [0][0][0][0]
    frames.frame.0.media_type="video"
    frames.frame.0.stream_index=0
    frames.frame.0.key_frame=0
    frames.frame.0.pts=34536
    frames.frame.0.pts_time="0.383733"
    frames.frame.0.pkt_dts=34536
    frames.frame.0.pkt_dts_time="0.383733"
    frames.frame.0.best_effort_timestamp=34536
    frames.frame.0.best_effort_timestamp_time="0.383733"
    frames.frame.0.pkt_duration=1501
    frames.frame.0.pkt_duration_time="0.016678"
    frames.frame.0.pkt_pos="1834827"
    frames.frame.0.pkt_size="14917"
    frames.frame.0.width=1920
    frames.frame.0.height=1080
    frames.frame.0.pix_fmt="yuv420p"
    frames.frame.0.sample_aspect_ratio="1:1"
    frames.frame.0.pict_type="P"
    frames.frame.0.coded_picture_number=120
    frames.frame.0.display_picture_number=0
    frames.frame.0.interlaced_frame=0
    frames.frame.0.top_field_first=0
    frames.frame.0.repeat_pict=0
    frames.frame.0.color_range="tv"
    frames.frame.0.color_space="bt709"
    frames.frame.0.color_primaries="unknown"
    frames.frame.0.color_transfer="bt709"
    frames.frame.0.chroma_location="left"
    frames.frame.0.tags.timecode="20:18:26:50"
    frames.frame.0.side_data_list.side_data.0.side_data_type="H.26[45] User Data Unregistered SEI message"
    frames.frame.0.side_data_list.side_data.1.side_data_type="H.26[45] User Data Unregistered SEI message"
    frames.frame.0.side_data_list.side_data.2.side_data_type="SMPTE 12-1 timecode"
    frames.frame.0.side_data_list.side_data.2.timecodes.timecode.0.value="20:18:26:50"
    frames.frame.1.media_type="video"
    frames.frame.1.stream_index=0
    frames.frame.1.key_frame=0
    frames.frame.1.pts=36036
    frames.frame.1.pts_time="0.400400"
    frames.frame.1.pkt_dts=36036
    frames.frame.1.pkt_dts_time="0.400400"
    frames.frame.1.best_effort_timestamp=36036
    frames.frame.1.best_effort_timestamp_time="0.400400"
    frames.frame.1.pkt_duration=1501
    frames.frame.1.pkt_duration_time="0.016678"
    frames.frame.1.pkt_pos="1857434"
    frames.frame.1.pkt_size="14472"
    frames.frame.1.width=1920
    frames.frame.1.height=1080
    frames.frame.1.pix_fmt="yuv420p"
    frames.frame.1.sample_aspect_ratio="1:1"
    frames.frame.1.pict_type="P"

As seen in this output, the 1st frame appears to be a P frame. I’m curious as to how will the decoder decode this frame without a preceding I frame (key frame)? The P frame will not have enough information to reconstruct the image from its residual data without the key frame.