I’m guessing it’s a Paragon pot? The interest rate reduces to 0.5% on 1st July, so – as I understand it – brand new customers (i.e. if you don’t have a Paragon pot now) will get that rate. You may be allowed to create one at 0.8% (because behind the scenes you’re just expanding the amount of money in the same account you already have with them, even though it looks like multiple pots in the Monzo app) but it will reduce on 1st July.
Edit: You should have received an email about this.