On ffmpeg version 7.0, using sendcmd doesn’t work with instance names.
Ideally I wouldn’t use the PARSED_x_filter syntax

Simple repro

import subprocess

input_vid = "../second_short.mp4"
output_vid = "tmp/second_face_follow.mp4"
send_cmd_file = "tmp/second_face_follow.txt"
duration = 17
with open(send_cmd_file, "w") as f:
    for i in range(duration):
        f.write(f"{i} crop_4 x {i*10};n")
    (9/16):in_h:0:0[pad_a];[0:v]crop@crop_1=in_h*(9/16):in_h:0:0,sendcmd=f={send_cmd_file}' -c:v h264_videotoolbox {output_vid}"
command = f"ffmpeg -y -i {input_vid} -filter_complex 'crop@crop_4=in_h*(9/16):in_h:0:0,sendcmd=f={send_cmd_file}' -c:v h264_videotoolbox {output_vid}"

subprocess.run(command, shell=True)

Am i messing up the syntax or is there a workaround where i can reference the filter instance names?