Does not work for me, unfortunately:
[hevc_vaapi @ 0x55e603611700] Driver does not support some wanted packed headers (wanted 0xd, found 0x1).
[mp4 @ 0x55e60365ea80] track 1: codec frame size is not set
[mp4 @ 0x55e60365ea80] Could not find tag for codec subrip in stream #2, codec not currently supported in container
Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
Error initializing output stream 0:0 --