I do not know if the Fairy type has been addressed in Eeveelutions yet, because I fell behind reading (I got to Introducing traditions, though, so not that behind), but reading Fire and Steel got me thinking.

If the dragons were born out of The Weave, I thought that maybe fairies were born into it. Tales and flights of fancy influencing the magic, subtly (or not-so-subtly) shaping it and becoming self-aware. Dragon energy shapes the rules by directing the user's will, while fey energy makes rules by taking advantage of what's already there - maybe you dig a small cove of your own and have things... fall into place and settle, as it were. The fabled glamour of the fae could be distorting the presence (or absence) of something by presenting the user's will as capital-R-Reality instead of forcing the issue as a dragon would. A dragon-type attack would, by this model, "ground" into the fairy and leave them unharmed, whereas a fairy would, say, borrow the symbolism of moonlight to hit the dragon with a Moonblast and metaphysically remind them of their place of origin.

Metal has always been portrayed as being resistant to magic in someway or another, usually by impeding its flow which is a common justification for why mages in some settings tend to forgo metal armor unless it's a very special metal. Or you could go the Exalted route and have it so that the material is somehow representative of the stability of the actual laws of reality, thus rejecting the raw Dragon-type energy trying to force a change, and doing the same to the manipulations of both the Fairy- and the Psychic-types; Fighting-types use Aura, which has different metaphysical rules that can act upon the metal with ease, much like how materials have inherent strengths and weakness that give them different uses.

I mean, glass lets light through, blocks the wind, but isn't terribly resilient. Plexiglas is another story, but that has its own set of properties, which include having different production methods.

What do you think?

if the Fairy type has been addressed in Eeveelutions yet

Afaik stuff like Hearts & Hooves days and Shining Armor's shield which made Abes feel sick have been the only things stated to be Fairy-type so far.