On my system it happened that x11-terms/kitty would select the wrong fallback font for emojis. A debugging session revealed that fc-list would not list NotoColorEmoji.ttf provided by media-fonts/noto-emoji. The fontconfig conf file /etc/fonts/conf.avail/70-no-bitmaps.conf was the only only providing a <rejectfont> match. After disabling 70-no-bitmaps.conf via eselect the NotoColorEmoji.ttf is now reported by fc-list (after fc-cache). This appears strange as NotoColorEmoji.ttf is not a bitmap font. Taking a closer look into the 70-no-bitmaps.conf we see this: <patelt name="outline"><bool>false</bool></patelt> In contrast, on a Debian Bookworm installation, the very same file contains <patelt name="scalable"><bool>false</bool></patelt> After changing "outline" to "scalable" an reactivating 70-no-bitmaps.conf, NotoColorEmoji.ttf is now reported, as it is on Debian. I suggest to change /etc/fonts/conf.avail/70-no-bitmaps.conf accordingly as the "scalable" property rather captures what we mean by "no-bitmap", I believe.
