In the “before descent” section you state you press the level change button twice…
Just me, but I never use the level change button and opt instead to “pull” the button to activate the change in the A/P. I do this by using the mouse to left click on the button and at the same time center click. Could this be your issue?
Again, I’ve spent many hours in the A310 and I’ve yet to use level change. Everything works as it should.