To reproduce:
- Open the https://start.jakarta.ee page - Jakarta EE 11 and Core Profile are selected:

2. Select Jakarta EE 10 - Web Profile gets selected automatically:

3. It's not possible to select Platform in the profile option:
I believe that, after Jakarta EE 10 is selected, Platform should be available in profiles and it even should be selected by default.
To work around this:
- Select some runtime - this allows selecting Platform
- Select None as runtime again - Platform remains available as expected