Thomas Juth flags a sneaky problem with automating vocals under heavy mix bus compression. If your multiband or mid-side bus compressor is working hard, it's likely ducking the mix every time the vocal hits, shaping the balance you're hearing while you automate.
Pull back that compression later and the whole vocal mix shifts, because the ducking that was propping up your levels disappears with it.
The fix is to settle your bus processing before locking in automation, not after, so the balance you dial in actually holds.