Altium->Coupled Inductors
What is wrong?


You've made the beginner's error of not understanding the importance of dot-notation when implementing a flyback transformer design. In your first circuit you have, in effect, got lucky with the right output voltage but it's not acting as a flyback converter but more like a regular forward converter and using the transformer as a regular step-down device.

This is not how flyback designs work.



The lower picture shows how I've altered the secondary coil to suit it working as a flyback converter. Flyback converters work by charging the primary coil and reverse biasing the secondary diode then, turning off the primary MOSFET so that "fly-back" occurs and the energy stored in the transformer's magnetic field is released and forward biases the diode. It's a two-phase operation.

Correct use of transformer secondary phase relative to primary phase: -



Notice the change in the position of the dot on the secondary - you can keep the dot as per your original diagram but you then need to connect the secondary diode to the non-dotted end of the secondary (as shown in my amendment to your original diagram).

Picture from A Guide to Flyback Transformers by CoilCraft.

Other reading: Mean Well - Flyback converter: -