(so she can complain about the help she just received...lol).
To be fair, that depends on how your initial interaction with her goes in the Teahouse.
If you tell her about Ethel mudering her brothers, her interaction deep in the lair is simply despair about almost giving Ethel her child and then being thankful to you for saving her.
The only thing that was slightly different in the fight this time around is that Ethel changed into Mayrina during the very last round - though I did "dispel" the illusion by hitting the fake Mayrina before the fight ended. But Ethel pulling that fake-Mayrina-trick happens pretty rarely for me and I don't think I've ever seen it happen so close to the fight's end - so maybe this had something to do with the glitch? Not sure.
That would be my guess. Since this move also teleports Mayrina out of her cage, it's possible the game bugs out when trying to locate her during the dialogue with Ethel and incidentally despawns her.