I have never seen it described anywhere, but I'm nevertheless convinced that dernier and premier are pronounced more as dernière and première, respectively, when they occur before a masculine noun that begins with a vowel or a mute h. ex. le dernier appel ; le dernier endroit ; le premier enfant ; le premier hôtel ; le premier étage ; le premier avril (the first of April).
It's so obvious that I thought they must be mistakes, but now I'm convinced that it must be some sort of unspoken (jeu de mots volontaire) convention.
Robert