Some instances disable downvotes, some do not. It’s a little confusing. I think lemmy.one is one of the instances (and beehaw and a few others) that have downvotes turned off.

Your instance is lemmy.one, which has downvotes disabled, Sync is detecting that and hiding the downvote button. Other instances that also have them disabled are beehaw, lemmy.blahaj.zone, and a few others.

In the other apps you’re seeing the downvote button because it isn’t checking if downvotes are disabled or not, if you press downvote in those apps it’ll look like it went through, but the downvote doesn’t actually happen.

There are very few instances that disable downvotes, so a lot of the apps still show the downvote button due to just not knowing it’s a thing

Edit: If an app dev happens to come by this and wants to add a check if downvotes are enabled in your app, the API to check is here: https://join-lemmy.org/api/interfaces/LocalSite.html#enable_downvotes