Concating file A with A, first A is silent

Off-Topic
16

BTW:

If I try it with:
ffmpeg -i "concat:dormi.mp3|4sec.mp3|dormi.mp3" -acodec copy dormi4secdormi_created-with-ffmpeg.mp3

I get this file (remove the additional .txt before playing)
dormi4secdormi_created-with-ffmpeg.mp3.txt (67,3 KB)