In BG3, nothing that boosts your stats is considered part of your base stat.
Most likely some sort of technical reason for how they made stat changes work.
But it leads to all sorts of jank, most notably stuff like the stat modifying items like Gloves of Dexterity or Elixir's of (X) Giant's Strength. Where in skill checks it'll show a -1 modifier from having 8 in their stat then a +4 or +6/+9 bonus from these effects.
But yeah, Ethel's Hair is simply a buff applied to your character not an actual alteration to base stats (You can see it listed in your character sheet). Thus it is not factored in on level up screens which only utilize your base character without any additional effects.
This is also likely what enables the mod that exists to change the stat granted by Ethel's Hair, since the mod only has to change which variant of the Ethel's Hair buff you have rather than manipulating base stats (Lowering the one you had already chosen and raising the one you newly selected)
