RDS PostgreSQL instance class modification don't take effect

Hello,

I'm trying to modify my RDS PostgreSQL instance class from "db.t3.2xlarge" to "db.m5.2xlarge". I make the change using the portal and check the option to apply immediately. The status is shown as "Modifying" but when completed, the instance class is not changed.

I tried also with other classes, nothing works.

PS: The instance class was "db.m5.2xlarge" at first, and since I changed it to "db.t3.2xlarge", I can't change it anymore.

Please help.