Something that I have noticed when fontmaking is that with a new project there is no grey glyph for semicolon in the glyph overview window.
Is this a bug?
I mention this because I am wondering what is causing the problem. I have checked the mappings and the postscript names with some other fonts and they seem fine.
If you don’t see a sample, then the “sample font” doesn’t contain that specific glyph itself, although that is very unlikely. So I suspect the glyph has misses the codepoint. In your case $3B, as that represents the semicolon character.
If I start a new project and don’t include outlines, there are no grey glyphs for semicolon, middle dot, fraction slash or hyphen-minus. The font used in the overview is Arial.
I have two glyphs: $2010 (hyphen) and $2011 (non-breaking hyphen) in my Template font, which are both composites of $2D (hyphen), which is also mapped to $AD (soft hyphen).
The first glyph ($2010) shows a grey outline in an empty glyph, but the second one ($2011) does not.
The hyphen glyph ($2D) in the template font is also mapped to the soft hyphen ($AD).
The soft hyphen and the hyphen both show grey glyphs in other fonts when empty.