This alias means that filtering out "solo_x" content, you lose all content with X.

So posts like this get hit:

https://e6ai.net/posts/181835

When what I really want to filter is posts like this:

https://e6ai.net/posts/181960

What you want to do is put solo x on one line of your blacklist without an underscore. This will filter out all posts that contain both of those tags on that line. The reason the alias is there is because it's not very useful to have something like solo_male as a tag by itself as it is functionally no different than a post having both solo and male tagged on it.

Edit: If you're trying to do multiple subjects/genders of solo posts, do different lines for each. Like,
solo male
solo gynomorph
solo andromorph

and so on.