Ideas
Edit: From further experience, I don't think it's caused by stick drift either, the cursor just starts at the top left of the screen.
Original: It is caused by stick drift. The issue is that the spawn screen UI doesn't respect your controller deadzone settings, so any slight amount of input will cause the cursor to move at full speed in that direction.
BF6's default deadzone setting is 8 and your left stick drifts less than that, but it still has a small amount of drift.
The spawn screen cursor also needs to have a smoother input curve to make small adjustments easier (small inputs should move the cursor slower), and selections need greater magnetism to improve ease and speed of selecting.
This is not stick drift it does it with every controller I use. I used three dualsense and 2 Xbox controllers and have the issue. This is an imput issue that I hope they fix.