I am surprised that this has not already been raised as an issue.
For me, the slightest downwards pull on my mouse button while clicking on a tab, drags the tab into a new window. And when you drag it back NONE of the tabs in Brave then work properly.
It is SO SO frustrating and although I have been persevering with Brave, this combined with other instability issues means I am almost at the point where I am going to give up and go back to Chrome - which I really do not want to do.