Because there is a buildup of protoporphyrin IX with no malfunction of the FECH enzyme, all the available Fe2+ is used in the production of heme, causing the FECH enzyme to use Zn2+ in its place,  causing a buildup of zinc-protoporphyrin IX.
The presence of excess protoporphyrin IX despite no FECH enzyme dysfunction indicates that there is insufficient iron (Fe2+) available for heme synthesis. This leads to the FECH enzyme switching to using zinc (Zn2+) as a substitute, resulting in an accumulation of zinc-protoporphyrin IX.