System Information
Operating system: Arch Linux
Graphics card: AMD Polaris 22 (DRM 3.42.0, 5.14.12-arch1-1, LLVM 12.0.1)

Blender Version
Broken: 2.93.5, commit date: 2021-10-05 12:04, hash: a791bdabd0b2

Short description of error

After importing VP9 video and trying to render I got blocky render.

Exact steps for others to reproduce the error

1. Loaded default environment and chose "Video Editing"
2. Uploaded video file
3. Made simple modifications (split, audio replacement) without touching video parameters
4. Render video

Source video information (from ffmpeg -i):

Input #0, matroska,webm, from 'filename.mkv':
  Metadata:
    ENCODER         : Lavf58.76.100
  Duration: 00:03:20.11, start: 0.000000, bitrate: 348 kb/s
  Stream #0:0(eng): Video: vp9 (Profile 0), yuv420p(tv), 480x360, SAR 1:1 DAR 4:3, 24.87 fps, 24.87 tbr, 1k tbn, 1k tbc (default)
    Metadata:
      DURATION        : 00:03:20.000000000
  Stream #0:1: Audio: aac (LC), 44100 Hz, stereo, fltp (default)
    Metadata:
      HANDLER_NAME    : SoundHandler
      VENDOR_ID       : [0][0][0][0]
      DURATION        : 00:03:20.109000000

I do not use accelerated rendering.

I tried to change output parameters for video:

• Raw PNG frames
• MPEG-4 + H.264 Good
• MPEG-4 + H.264 Loseless
• MKV + H.264 Good
• WebM + VP9 Good

Every time render gets borked. Preview shows original quality which isn't as bad as seen on screenshots.

When I converted source from MKV+VP9 to MPEG-4+H.264 blender finished render without artifacts.