Hey 
I’m a savings engineer here at Monzo, sadly we don’t control the specific savings product you are on the rates are defined by our partners and differ from the latest rates. This goes both ways during the pandemic older customers were getting better rates than new customers, now that we are trying to get back to the new normal rates are increasing and our partners are raising rates for both our latest products available for new customers and normally for older products too.
These might not happen at the same time but we are always talking with our partners to try and get the best deals we can for our customers.
I hope this helps explain why these rates differ to what is available from new customers straight away.