I don't get the Deltarune comparison, but I feel like I understand exactly what you mean. I'm a woman. The way I act and the way I dress are more masculine than is typically expected of a woman. It's only after I started dressing "like a man" and cutting my hair short and not suppressing more "masculine" mannerisms that I started to feel like myself. I've used resources designed for trans men like style and fashion guides, but I've thought about it a lot and the fact does remain that I'm a woman.

People generally expect that the way you (want to) look/dress/act is to a large degree aligned with the typical presentation of your gender in your cisheteronormative society, and they have a hard time wrapping their heads around it when you don't meet that threshold. This is mostly the case in LGBTQ spaces as well, in my experience. Some people in LGBTQ spaces will see aspects of my gender presentation (for example, I sometimes bind my chest) and take it as evidence that I'm in the early stages of coming out as a trans man. It's frustrating that in order for people to see me as my gender, I have to present in a way that makes me feel like I'm not myself. It's like either I can see myself as a woman, or I can get other people to see me as a woman, but not both at the same time.