Creating iPhone compatible video with ffmpeg from an image and MP3 file

I have the following command that I have used to create an MP4 video file from an image and an MP3 file, it plays fine in a Chrome browser on desktop and on an Android phone - but it doesn't work on an iPhone 10. Do I need some extra codec or setting to make this compatible for iPhone? Should I be converting this into a MPEG file instead for wider compatibility?

ffmpeg -loop 1 -i 6f4aa5dfefc4dd32186f41315ad9d1e2-0.png -i "music.mp3" -c:v libx264 -tune stillimage -c:a aac -b:a 192k -pix_fmt yuv420p -shortest aa-image0.mp4

Here are the ffprobe details for the input MP3 file used to create the MP4 video and also the ffprobe for that output file:

Input #0, wav, from 'download0.mp3':
  Duration: 00:00:35.94, bitrate: 384 kb/s
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 24000 Hz, 1 channels, s16, 384 kb/s
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'aa-image0.mp4':
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.22.100
  Duration: 00:00:38.76, start: 0.000000, bitrate: 505 kb/s
    Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 2664x1498 [SAR 1:1 DAR 1332:749], 367 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, mono, fltp, 138 kb/s (default)
      handler_name    : SoundHandler

The video seems to play on the iPhone, but the audio seems to start/stop constantly. Like it is a streaming issue (even though it is not). Maybe an MP3 encoding issue when I created the video?

Here are the test files I have used and created: (OUTPUT)