In FontLab, remove “Mathematical Operators” from the Supported Codepages list, and regenerate the font.

Unfortunately, the supported codepages are 1252 Latin 1 and 1250 Latin 2

I am getting closer to solving this for sure. At the moment I have a short list of likely culprits. Removing “Mathematical Operators” is not on the list because I have a font that has that and it shows letters in the Windows icon.

I have

Basic Latin
Latin 1 Supplement
Latin Extended A
Latin Extended B
Spacing modifier Letters
Greek
Latin Extended additional
General Punctuation
Currency Symbols
Letterlike symbols
Mathematical Operators
Private Use
Alphabetic Presentation forms
Geometric Shapes

Perhaps you can A/B test enough of these to get an answer faster than I can just now. I don't think all these are equally likely ( Greek? ...probably not ) but I can not say with authority which one makes the difference only that the key check box is probably in this set. That is assuming of course that there is only one! You might need more than one.

Found it. In the Fontlab info section section "Encoding and Unicode", subsection "Unicode Ranges", UNcheck the box labeled "Mathematical Operators".

Really peculiar.

Found it by viewing with thumbnails ( I usually don't) a random folder of ttf fonts and found a couple with the same symptom and looked into them; generated them with and without that flag set and the thumbnails changed.

- Herb

@HVB Unfortunately it is unchecked :(

I have tested fonts with & without Mathematical Operators. It isn't the one. One of the ones I listed is the one. I just don't know which one yet. Maybe "Alphabetic Presentation forms" or "Letterlike symbols".

@oldnick you were right. I don't why why it didn't happen the first time. But today I incrementally copied the font info of my current font to a dummy font and then I started generating font(from the dummy font) after copying each page from my original font font. Guess what? Turing off mathematical operators check box cured the issue.
Thanks
Typogruffer