Yes, that is intended Every bank level has a specific interest percentage and a maximum amount of interest.
With your bank level, 1.350.000 credits is the highest amount of credits you can earn per day. If you upgrade your bank, you can earn more, but it has a lower interest %, so it requires more credits to reach the same amount of interest.
So if you want to earn more interest, then you have to upgrade your bank.
Every bank level has a specific interest percentage and a maximum amount of interest.