This may be an issue with your keyframe interval and fps settings from the encoder. Chunks must start on a keyframe, so it's best to use a keyframe interval that's a factor of the cupertinoChunkDurationTarget setting. For example, if cupertinoChunkDurationTarget is set to 10 seconds, use a keyframe interval of either 2, 2.5, 5, or 10 seconds.
You might try smaller cupertinoChunkDurationTarget. Set the encoder keyframe interval to 30 with fps at 15 (2 second keyframe interval). Then you can set cupertinoChunkDurationTarget as low as 2000, each chunk will be 2 seconds, which 1/5 the size of your 10 second chunks. That should be more manageable for low bandwidth clients.