I think you are on to it! Because I just tried to turn first person on right now to see what happens and the problem came back. But after I disabled it, the problem continued… I had to disable First person AND double click on the background after it!
I think you’ve got the right solution already. But occasionally I’ve found that the navigation goes crazy if I’ve done some extreme scaling of an object using gizmo - if you go to gizmo settings and bake the scaling everything calms down again!