To be fair Vader didn’t sense her death
But he did. That’s why he said “Where is Padme? Is she safe? Is she alright?” and “She was alive! I felt it!”
When he was being operated on, he could feel her presence, which is also why he said “Padme help me” as the mask was being lowered on him. But after the operation, she died and he couldn’t feel her anymore, which is why he was confused.
At least that’s how I interpreted it.