I attended the Snapcraft Clinic yesterday, and the suggestion I got sent me on a investigation that helped rule out problems with FONTCONFIG_PATH etc as the desktop-launch script Snippet Pixie already uses does the right thing.
However, it was also mentioned to try out another GTK snap to see if it worked and check out its scripts etc. So I tried @popey’s recent spot snap. It worked fine. Looking at it, although confined unlike Snippet Pixie which is classic, it’s also using core20 for its base.
So I’ve just switched Snippet Pixie from core18 to also use a base of core20, and guess what…
… it’s working! 
It’s currently in the candidate channel if anyone wants to give it a spin (I want to give it a couple of days and try it out on a few distros before promoting to stable).
Thanks @popey and (I think) @Igor for the suggestions during the clinic, and to @mborzecki again for the help above.