So this was a combination of me forgetting to add 'emoji' into the
list of 'simple' scripts in fontset.el, and the following code in
/* Don't use a color bitmap font unless its family is
explicitly specified. */
if ((sym_traits & kCTFontTraitColorGlyphs) && NILP (family))
Yamamoto-san, Iʼm not sure I understand the rationale for excluding
colour fonts unless explicitly requested, could you elaborate?
I moved away from macOS several months ago, but if memory serves, this was (at the time) a feature that only worked on non-free OSs, and a request was made (from Richard, if memory serves twice) to limit the feature until it could be adequately supported on free/libre systems.
If I'm barking up the wrong tree, please accept my apologies in advance.