iframe_playlist for fmp4 doesn't contain all keyframes

[SteveR-PMP]

System info

Operating System: Ubuntu 20.04
Shaka Packager Version: 3.4.1 (latest)

Issue and steps to reproduce the problem

Packager Command:
build/packager/packager 'in=packager/media/test/data/bear-1280x720.mp4,stream=video,output=/tmp/shaka/test/bear.mp4,iframe_playlist_name=stream_iframe.m3u8' --hls_master_playlist_output /tmp/shaka/test/master.m3u8

Extra steps to reproduce the problem?
This is using the sample mp4 files included with shaka that only have a few key frames but still will demonstrate the problem.

What is the expected result?
The resulting stream_iframe.m3u8 should contain an entry for each keyframe as it does with ts segment as shown here:
build/packager/packager 'in=packager/media/test/data/bear-1280x720.mp4,stream=video,segment_template=/tmp/shaka/test/seg_$Number$.ts,iframe_playlist_name=stream_iframe.m3u8' --hls_master_playlist_output /tmp/shaka/test/master.m3u8

#EXT-X-I-FRAMES-ONLY
#EXTINF:1.001,
#EXT-X-BYTERANGE:31584@376
seg_1.ts
#EXTINF:1.001,
#EXT-X-BYTERANGE:38352@276172
seg_1.ts
#EXTINF:0.734,
#EXT-X-BYTERANGE:44180@615324
seg_1.ts

What happens instead?
I only get an entry for the initial keyframe of a segment.
#EXT-X-I-FRAMES-ONLY
#EXT-X-MAP:URI="bear.mp4",BYTERANGE="866@0"
#EXTINF:2.736,
#EXT-X-BYTERANGE:31903@910
bear.mp4

<Please attach the input files or email to [email protected].>