but even ᴡindows® Notepad tries to substitute unknown characters from glyphs in other fonts instead of using the replacement character directly. Sometimes it even substitute characters whereas they exist in the font

Mono doesn't have that glyph, so what you're seeing is that that symbol is being used from another font to be able to show something

No, there is no “glyph not found” character. Different programs use different graphic presentations. An empty narrow rectangle is a common rendering, but not the only one. It could also be a rectangle with a question mark in it or with the code number of the character, in hexadecimal, in it.

The glyph-not-found character is specified by the font engine and by the font; there is no fixed character for it.

missing glyph

There is no standardized look/glyph, it’s up to the implementation

A character exists, but the glyph to display it isn't available.