Form not picking up dynamically displayed elements

[Bartmax]

I was playing with a dynamic form, and I have one field dependant on another.
When changing platform to Steam, it enables the platform selection, which works fine.
If I change the value of the Platform InputSelect and post the form work as expected, but
if I use the default value (aka not changing the first value by user interaction) the value submitted by the form is null.

Is this a bug or working as design ?
What would be a good approach so that when the SelectInput is render into the UI the form picks it's value ?

        <fieldset class="form-group">
            <label>
                Platform
                <InputSelect class="form-control" @bind-Value="ticket.Platform">
                    <option>iOS</option>
                    <option>Android</option>
                    <option>Steam</option>
                </InputSelect>
            </label>
        </fieldset>
        @if (ticket.Platform == "Steam")
        {
        <fieldset class="form-group">
            <label>
                OS
                <InputSelect class="form-control" @bind-Value="ticket.OS">
                    <option>Windows</option>
                    <option>Mac OS</option>
                    <option>Linux</option>
                </InputSelect>
            </label>
        </fieldset>
        }

[Bartmax]

I think my issue is about dealing with null select.
Switching mental model from HTML form to C# Model made it clear.

This model represents more clearly the intent and also works with that HTML

        private string _os = "Windows";
        public string? OS
        {
            get => Platform == "Steam" ? _os : null;
            set => _os = value ?? "Windows";
        }