Sync only happens if you intentionally do it. It’s not something that can “accidentally” happen. You literally have to produce a code that is individual to you and then input it on the other device. Without doing that, there’s no way the browsers on different device could communicate.
I’d be curious to see this and hear what OS you’re seeing it on. This isn’t something I’ve ever seen from Brave. Makes me wonder if you have a legitimate version of Brave installed or what is going on.