webm conversion fails when going from 4:2:2 to 4:2:0 subsampling

[frozenpandaman]

mpv version and platform

macOS 12.6.3 (M1/ARM architecture)

mpv 0.35.0 Copyright © 2000-2022 mpv/MPlayer/mplayer2 projects
 built on Sat Nov 12 14:47:29 UTC 2022
FFmpeg library versions:
   libavutil       57.28.100
   libavcodec      59.37.100
   libavformat     59.27.100
   libswscale      6.7.100
   libavfilter     8.44.100
   libswresample   4.7.100
FFmpeg version: 5.1.2

Description & log

I think this (title) is what's going on – webm conversion fails completely, whether using two-pass or not, and mp4 conversion scales the subs very strangely.

[  17.027][e][webm] Assertion failed: (x1 <= img->w && y1 <= img->h), function mp_image_crop, file mp_image.c, line 552.
[  17.027][v][webm] Command stdout:  
[  17.027][v][webm]  (+) Video --vid=1 (*) (h264 1920x1080 23.976fps)
[  17.027][v][webm]  (+) Audio --aid=1 --alang=jpn (*) (aac 2ch 48000Hz)
[  17.027][v][webm]  (+) Subs  --sid=1 --slang=ind (*) 'HAM' (ass)
[  17.027][v][webm] [autoconvert] Converting yuv422p -> yuv420p
[  17.027][v][webm]  
[  17.027][v][webm] Command failed! Reason:  unknown  Killed by us?  no