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.
Due to an accumulation of protoporphyrin IX without FECH enzyme malfunction, all available Fe2+ is utilized in heme production, forcing the FECH enzyme to employ Zn2+ instead, resulting in a buildup of zinc-protoporphyrin IX.