After changing the sample text on fonts.bunny.net to something that is non-English, all missing characters are all rendered with the same font. Chrome inspector will show the actual font being used under "Rendered fonts", and it appears to be whatever the locally configured fallback is.

This is in contrast with the behavior on fonts.google.com where missing characters are rendered with an inline image to explicitly show the missing glyph.

I prefer the fonts.google.com behavior here, which makes it easier to find fonts that have all the glyphs I need.