There are no negative likes in Discourse core.
That’s a big part of why they aren’t in Discourse core.
Whoever is running whatever site it is, is using some plugin that implements those. It’s pretty likely that it’s working as expected.
You’ll need to convince the site admins that it’s a problem that they want fixed. If it affects only the user who is downgrading the post, it doesn’t seem like much of a bug.