Cannot find a matching stream for unlabeled input pad 1 on filter Parsed_concat_0 (a different case)

I saw this answer ffmpeg batch overlay Error: "Cannot find a matching stream for unlabeled input pad 1 on filter Parsed_overlay_0" for same error message but doesn’t seem to be same case :

I first converted 2 gifs of same width and height to mp4 using ffmpeg options

ffmpeg -i video1.gif -movflags faststart -pix_fmt yuv420p -vf "scale=trunc(iw/2)*2:trunc(ih/2)*2" video1.mp4
ffmpeg -i video2.gif -movflags faststart -pix_fmt yuv420p -vf "scale=trunc(iw/2)*2:trunc(ih/2)*2" video2.mp4

afterwards I joined with

ffmpeg -i video1.mp4 -i video2.mp4 -filter_complex "concat=n=2:v=1:a=1" -f MP4 -y video.mp4

But I got error

Cannot find a matching stream for unlabeled input pad 1 on filter Parsed_concat_0

with other mp4 in the past joining works so I don’t understand why.