This is correct to happen like this. The reason this happened is the Canada Workers Benefit (CWB) was automatically applied previously in your return due to low income, therefore, increasing your refund. When you included your tuition, the software detected that you were a student and removed the CWB as full time students are not usually eligible to receive the CWB.
Thank you for choosing TurboTax.
You have clicked a link to a site outside of the TurboTax Community. By clicking "Continue", you will leave the Community and be taken to that site instead.