In directwrite fallback script list for shaping is DFLT -> dflt -> latn in descending priority, when explicitly set script is not available. I'm not saying that gdi32 should do the same, but DFLT vs dflt should likely be both supported, same as harfbuzz does. DFLT is the correct one, while 'dflt' is claimed to come from misleading Uniscribe documentation, and some fonts are using that.