After sharing my library for displaying ILBM/IFF files, Amiga Graphics correctly pointed out that the HAM image handling was buggy, this is now fixed: I was not making a copy of the palette colour used in Hold & Modify, so it was getting modified, i.e. corrupted.
The following image, Angel, by Dennis Jacobson/JAKE Productions is a ILBM/IFF Hold and Modify image based on a picture of Karen Velez from 1985. Note that the source pixmap is 400 × 320 pixels with a rendering ratio of 20 ÷ 11, the Amiga had strange video modes.
Just because an IFF file is 400×320 it doesn’t mean that the video screen was 400×320. Also, remember that the Amiga was one of the few computers of the era which supported overscanning (rendering video in areas that were otherwise considered “borders” by most computers), which is one of the reasons why the Amiga had a large presence in video production companies.