When expanding the monphone system to triphone system, I encountered this problem.
From the page : http://hts.sp.nitech.ac.jp/hts-users/spool/2009/msg00271.html, it suggests to check the mktri.hed file, where I found the expanding of sp as triphones.
Thus the problem is in the process of generating the mktri.hed file, which is done using:
perl maketrihed [mono_list] triphn_seen.lst
The correct way of doing so is:
perl maketrihed [mono_no_sp_no_sil_list] triphn_seen.lst