If there is a flammable block adjacent to the space next to a campfire, using flint and steel on that side of the campfire sets fire to the block instead. This applies when using flint and steel on the side of the campfire if there is a flammable block below and to the side (e.g. if the campfire is on a flat surface of grass blocks). It also applies when using flint and steel on the top surface of the campfire if there is a flammable block above and to the side (attached are photos of this situation, where flammable blocks get set on fire and non-flammable blocks allow the campfire to be lit).

Lighting the campfire should take priority if the targeted face is a campfire block, even if there are other flammable blocks adjacent to the targeted space.