DD

There are many reasons why a baby stops growing, most of them are genetic. Many of these genetic problems will have no effect on the heart. Therefore the heart can continue to beat even though the baby is not growing.
A slow heartbat is a sign that something may be wrong. You say that the hartbeat of your current pregnancy sems faster than your MC. A normal heartbeat should be between 110 and 160 and is usually closer to 160 than 110. When you hear this for the first it often seems quite fast
I hope this answers your question.