Animated GIFs are handled differently from normal "images". In fact, they're more like actual videos. That's why you can't use the image demuxer (i.e., can't use the -pattern_type
option) there.
What you need to do is convert each one individually to an intermediate file with the same properties, then concatenate those.
ffmpeg -i gif1.gif -vf "…" -r 25 -c:v ffv1 output1.avi ffmpeg -i gif2.gif -vf "…" -r 25 -c:v ffv1 output2.avi … ffmpeg -i "concat:output1.avi|output2.avi" -c:v libx264 output.mp4