mp4 frag files with 0 content in youtube dl download

I downloaded a video with youtube dl. My youtube DL is portable in case that matters. There was no sound and several part files - mp4 frag files
with 0 content. This was in the command box

"WARNING: XXXXXXXX: malformed AAC bitsteam detected. Install ffmpeg or avconv to fix this automatically". I put FFMpeg in the same folder as youtube dl and it played. It still played when i deleted the part files. Got another video and had sound but also part one file no content. They are
This is the mediainfo for the first. Why would it make part files with no content. Presume it is OK to delete these in all circumstances?
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General
ID : 16727 (0x4157)
Complete name : C:\Users\youtube-dl\movie.mp4
Format : MPEG-TS
File size : 1.04 GiB
Duration : 49 min 40 s
Overall bit rate mode : Variable
Overall bit rate : 4 933 kb/s

Video
ID : 33 (0x21)
Menu ID : 16727 (0x4157)
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L3.2
Format settings, CABAC : Yes
Format settings, ReFrames : 4 frames
Codec ID : 27
Duration : 49 min 40 s
Maximum bit rate : 5 070 kb/s
Width : 1 280 pixels
Height : 720 pixels
Display aspect ratio : 16:9
Frame rate : 50.000 FPS
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Color range : Limited
Color primaries : BT.709
Transfer characteristics : BT.709
Matrix coefficients : BT.709

Audio
ID : 34 (0x22)
Menu ID : 16727 (0x4157)
Format : AAC
Format/Info : Advanced Audio Codec
Format version : Version 4
Format profile : LC
Muxing mode : ADTS
Codec ID : 15
Duration : 49 min 40 s
Bit rate mode : Variable
Maximum bit rate : 320 kb/s
Channel(s) : 2 channels
Channel positions : Front: L R
Sampling rate : 48.0 kHz
Frame rate : 46.875 FPS (1024 spf)
Compression mode : Lossy
Delay relative to video : 20 ms
Language : English

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