I am adding new glyphs to an existing font but when I use the font (tried using it as desktop font on Linux and in text documents with LibreOffice) the newly added glyphs have slightly different interpretation of the shapes and at some sizes are of different height compared to the preexisting ones.
The newly added glyphs are non-unicode and intended for usage as single substitution local alternatives. The replacement of the glyphs when the font is in use works fine.
The problem occurs whether I generate fonts in *.ttf or *.otf, with or without 'autohint' option set, with or without autohinting and autoinstructing in the *.sfd and the *.ttf/*.otf files. I tried generating *.ttf without instructions and then using ttfautohint - same results.