It stays at 90° by design — there’s no adjustment.
The fence base is machined square to the table, so when you clamp it down it self-squares.
The second knob just locks the aluminum extrusion in place so it stays tight and doesn’t move.
If the fence isn’t at 90°, it’s usually because it wasn’t fully seated, there’s dust/debris on the table edge, or it wasn’t clamped evenly. The fence itself is designed to self-square and has no adjustment.