Forte (pronounced: fort) is the feminine version of fort (pronounced "forr").
Since fort/forte is an adjective (adjectif en français) it describes the noun (in this case 'La bière') and therefore must agree with the subject.
"Le papier est fort." - The paper is strong. "La table est fotre" - The table is strong.
This error is nearly 4 years old!
Just for the sake of accuracy, "forté" cannot be a past participle for there is no matching verb.
Yet, "forte", pronounced "fortay" is used in French as a word borrowed from Italian to qualify music: https://fr.wikipedia.org/wiki/Liste_des_termes_italiens_employ%C3%A9s_en_musique
Ordinarily, the ‘e’ at the end of ‘forte’ here would be voicelessly elided after the release of the [t]. When voiced for clarity or emphasis, most native French speakers would pronounce the vowel it as a mid-central rounded schwa [ɵ] or at least an unrounded schwa [ə]. Although the pronunciation of the French schwa phoneme /ə/ is highly variable, the front central pronunciation [e] we hear here is definitely unusual.