When defeating Loremaster there's some mistake in determining which Black Bird's voice to play and which objective to give to player. Regardless of the story branch player chosen (that is of how much Sigil pieces player already has), game gives him objective "Return back to Oracle" (and corresponding voice is played). In the original game, if player already killed Oracle and completes Sigil, he receives following objective:

You wield the power of the complete sigil. What do you say we get some closure. Let's see what the loremaster been protecting.

NOTE: I checked the progs, and surprisingly there IS a condition for this case in SpectralMonster::A_AlienSpectreDeath(), so I have no idea what can be wrong there. Can it be GetSigilPieces() method returns wrong number? NOTE2: This bug is purely cosmetic and does not prevent player from finishing game and getting correct ending sequence.