Not-an-anything but tinkered with custom fonts a bit. I didn't have any issue replacing a couple of (grayscale) glyphs (fire / U+1F525, and upwards arrow / U+2191 ). So I wonder if it's something else for you.
I did have an issue with double glyphs (and this gave an error too) but it was at higher ranges (with eye / U+1F441) and not even with a custom glyph.
I don't know if it makes a difference, but the fallback font I made with FontForge. Also I did a few normal letters, too:

Also (not shown), I made a color override script (e.g. turn the upwards arrow green) but that only works for 2D text (uses RichTextEffects, a bit clunky IMO). That, and also color emojis are a bit wonky too, only 2D and outlines a bit off but that may be a font issue.
EDIT: Technically it wasn't a fallback font because the letters are part of it. But I just tested it as one (with Cantarell as the SystemFont) and the fire glyph still works. Tried with your listed font too (FontVariation, though), so it must be an export or import issue with your glyphs (or some other setting issue).