Sounds like you qualify for the 300 more.
Your 2020 return starts out by assuming you didn't get any Stimulus payments so it gives you credit for the full amount and your refund was too high. Then near the end you enter how much you actually got so it only gives you the difference if any. So you don't get it again.
That's why you can see your refund or tax due change when you enter the Stimulus payments.
