I think "plothole" is not really the right word here. We got an explanation, but in a wrong way. That's because the writer decided to change the story in the last few chapters (according to some comments. I didn't translate the story). Seems like he had to wrap up the story really fast, so I think you are right about the axing part. It makes sense...
So instead of exploring why or how the ntr stuff happened, we just got "she is a nymphomaniac cheater who fakes memory loss", or smth like that. Although I think the drug part is real, that explains the tattoo and the box in the beginning, probably. But I didn't really bother translate the story, so I am not sure.
It's a shame, because there was so much stuff that had to be explained or explored. Instead it just ends suddenly with a threesome.
Should have been 100 chapters or so, lmao. Everything happened too fast.