Well she's accusing her of cheating, but we don't know if she's cheating or not at this point. It still could turn out that she's not, but adding tag would ruin that suspension. This way you can still read it hoping it'll turn out she was telling the truth about not cheating (and she was).
I think spoiling it for a few is a small sacrifice considering how much trauma the subject matter might bring up with people.
Unrelated to above, something about this story has been bugging me, besides the obvious. Second comment on this thread (now deleted it seems) mentioned multiple personalities, which would fit in mental illness category. Obviously if that was the aim of this oneshot then it failed to bring it across miserably, however one detail makes me think.
https://dynasty-scans.com/chapters/lovely_mental_illness_distortion#7 the name on the door. She's an adult woman, who lives with her lover, why would she have her own room? If she does, why would she have her name on the door like kids usually do? This might be a nod that the cheating is actually happening in her head between two of her personalities.
I don't know, it's a stretch, and as I said if that was the aim, then the author miserably failed to make the connection clearer, but I think assuming this angle is the only way I can keep myself from almost throwing up when this oneshot pops into my head.