Freetype also doesn't do the right thing on RGBW at least. Reading code in the repo suggests it's possible to reconfigure it to work properly with the Harmony algorithm but I haven't worked out how yet. If anyone knows how to sponsor efforts to fix this I would totally contribute to that.

A separate problem is that I don't think there is a standard way for monitors to communicate the subpixel layout in such a way the font rendering engine will have access to it. That seems like a pretty big oversight when introducing these in the first place.