Transparency ignored in drawbox filter when using the overlay filter

[est77]

When using drawbox,transparency is ignored if the overlay filter is used later in the filter chain. This used to work in version 2.8 but it does not work anymore in 3.2 or master.

How to reproduce:

# Transparency ignored:
ffmpeg -thread_queue_size 512 -hwaccel auto -threads 0 -y -probesize 5000000 -r 25 -start_number 1 -i "bg.001.png" -i "logo.png" -filter_complex "[0:v]null,scale=960x540,drawbox=x=-52.8181818182:y=-1:w=1063.63636364:h=540:t=50:c=#000000@0.75[clip];[1:v]scale=iw*0.21:ih*0.21[logo];[clip][logo]overlay=851.05:1.05[clip];[clip]scale=960x540, fps=25, setdar=1.77777777778,setpts=PTS-STARTPTS[v]" -map [v] result_with_logo.png

# Correct transparency (but no overlay)
ffmpeg -thread_queue_size 512 -hwaccel auto -threads 0 -y -probesize 5000000 -r 25 -start_number 1 -i "bg.001.png" -filter_complex "[0:v]null,scale=960x540,drawbox=x=-52.8181818182:y=-1:w=1063.63636364:h=540:t=50:c=#000000@0.75,scale=960x540, fps=25, setdar=1.77777777778,setpts=PTS-STARTPTS[v]" -map [v] result_without_logo.png

[est77]

Files needed to reproduce and results

[Elon Musk]

This is because drawbox can now draw alpha directly. Use format=yuv444p after drawbox to avoid that.