This is a technical limitation on the Amiga. The display actually consists of three 'screens', each with its own palette. The credits screen has to be separate from the gameplay screen because it fades independently. It takes a certain number of scanlines for the copper to change palette for each screen change meaning you need a few black lines which is where you may see a gameplay sprite get cut off.
The original Turrican 2 does the same thing, although as it only uses 16 colours it can do the screen swap in 1 scanline.