2 pass CRF: much higher bitrate if auto-alt-ref enabled

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phantom....@gmail.com

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Mar 13, 2020, 4:04:03 AM3/13/20
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ffmpeg -i "tos.y4m" -c:v libvpx-vp9 -quality good -cpu-used 2 -g 500 -auto-alt-ref 1 -tile-columns 1 -lag-in-frames 25 -aq-mode 0 -b:v 0 -crf 21 -threads 4 -row-mt 1 -enable-tpl 1 -pass 1 -y -f webm "tos2pass.mkv"

ffmpeg
-i "tos.y4m" -c:v libvpx-vp9 -quality good -cpu-used 2 -g 500 -auto-alt-ref 1 -tile-columns 1 -lag-in-frames 25 -aq-mode 0 -b:v 0 -crf 21 -threads 4 -row-mt 1 -enable-tpl 1 -pass 2 -y -f webm "tos2pass.mkv"


auto-alt-ref is disabled. 
encoding results:
frame=  576 fps=7.2 q=0.0 Lsize=    2888kB time=00:00:23.95 bitrate= 987.4kbits/s speed=0.298x

ffmpeg -i "tos.y4m" -c:v libvpx-vp9 -quality good -cpu-used 2 -g 500 -auto-alt-ref 0 -tile-columns 1 -lag-in-frames 25 -aq-mode 0 -b:v 0 -crf 21 -threads 4 -row-mt 1 -enable-tpl 1 -pass 1 -y -f webm "tos2pass-noaltref.mkv"

ffmpeg
-i "tos.y4m" -c:v libvpx-vp9 -quality good -cpu-used 2 -g 500 -auto-alt-ref 0 -tile-columns 1 -lag-in-frames 25 -aq-mode 0 -b:v 0 -crf 21 -threads 4 -row-mt 1 -enable-tpl 1 -pass 2 -y -f webm "tos2pass-noaltref.mkv"


auto-alt-ref is enabled. 
encoding results:
frame=  576 fps= 11 q=0.0 Lsize=    2234kB time=00:00:23.95 bitrate= 763.8kbits/s speed=0.451x

Is it bug or feature?
I thought that auto-alt-ref is a feature to increase compression performance (decrease bitrate) to make video files smaller.
But tests shows that enabling auto-alt-ref increase bitrate. 
What a auto-alt-ref is? Which cases should I enable it?


ffmpeg-20200101-7b58702-win64-static
ffmpeg version git-2020-01-01-7b58702
libvpx-vp9 @ 000001a26169d480
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