Sync has unfortunately been abandoned by the developer. Even if you're able to find a solution for this particular problem, it still might be worth checking out other apps so you'll have a few options in mind once Sync breaks completely at some point down the road.

I personally ended up swapping to Summit; it feels a lot like Sync and the dev is very responsive. But there are a lot of good options to choose from!

Sync is now unsynced by the dev, find a new app to use.

Tap the tree dots for Post Options and then tap where the arrow points in the pic I link.

I have the same issue with other apps. Voyager, Boost in android and iOS.