I got it to happen twice, the first instance was during an Emma event where she confesses her love to you and you have sex in her room, so she's in a "missionary-esque" pose that I think is supposed to be cowgirl, because it doesn't look like the standard missionary pose. The second time I believe was actually missionary. Both times when the bug occured sex had already been initiated, and scrolling back and going through again just repeats it. I can 'disengage' from Kitty and resume with Emma and Kitty will just stand in the background
EDIT: I did have cheats activated through the console commands
EDIT2: Happened again, this time in the shower, halfway through 'midday', vaginal missionary, Kitty was in a different outfit. Seems to be overlapped, because upon disengaging from Kitty, I'm already 'engaged' back with Emma as if Kitty had never shown up; I don't have to click anything, just back to 'mid-coitus', upon getting Emma to orgasm, Kitty disappears