Baby heartbeat decrease?

At 7w4d one of the twins heartbeat was 156bpm and the other is 144bpm, at 9w1d one of them heart stopped and the other is 128bpm,

Is it normal for a heartbeat to decrease?

I couldn’t talk or discuss much with my doctor due to i was in shock and speechless and she asked to see me after a week which is this Tuesday because the other twin didn’t vanish yet I haven’t notice the heart rate until i got home so of course I’ll talk to her about it on my next appointment but I’m wondering now if anyone have any information or experience