Post any defects you find in the released or beta versions of the ImageMagick software here. Include the ImageMagick version, OS, and any command-line required to reproduce the problem. Got a patch for a bug? Post it here.
You have specified a very small size for that font, so small that some verticals are less than half a pixel thick. So if you anti-alias them the verticals are gray but visible. If you don't anti-alias, they are zero thickness.
The font still doesn't look right at a larger size , but I shouldn't need to set a large size anyway. Terminus-(TTF) looks fine in gimp at 12pt. I think that this is a bug in ImageMagick. On the project page for Terminus TTF (https://files.ax86.net/terminus-ttf/), it says:
There are applications that neither support the original Terminus Font nor use bitmaps embedded in TrueType fonts. They completely rely on the automatically generated scalable outlines.
Another bitmap font I tried had the same problem. Maybe ImageMagick lacks support for bitmaps inside TTF fonts?