It is true the cooker should be on a separate circuit. But there is often also a ‘whole house’ RCD, common to all circuits. Small leakage currents in the cooker will then trip the ‘whole house’ RCD, without affecting the cooker’s fuse or MCB. That is correct operation.
The cooker may also be on a circuit that by-passes the main RCD, or the cooker circuit may have a separate RCD that’s comined with the MCB (RCBO), but I don’t think that’s very common. Or if more than 10-20 years old, there may be no RCD at all, but in that case it’d be worth getting a wiring update.
Incidentally RCDs need regular testing, and should have a label attached to that effect, you press a button that simulates a fault, to make it trip. I recently tested the one in my own fusebox and it didn’t work, no longer providing protection, I had to get an Electrician in to replace it...