dot:ComboBox, and probably any other controls that derive from Selector, do not support (nullable) boolean properties. Selecting the option with the value null behaves as expected, but selecting and other option will always set the value to true.

Using DotVVM.AspNetCore version 4.2.6 on net6.0.

Example code:

<div class="form-group" Validator.InvalidCssClass="has-error" Validator.Value="{value: Form.HasGenerator}">
    <label for="HasGenerator">Is there a generator?&nbsp;<span class="text-danger">*</span></label>
    <dot:ComboBox html:id="HasGenerator" SelectedValue="{value: Form.HasGenerator}" class="form-control">
        <dot:SelectorItem Text="Choose an option..." Value="{value: null}" />
        <dot:SelectorItem Text="No" Value="{value: false}" />
        <dot:SelectorItem Text="Yes" Value="{value: true}" />
    </dot:ComboBox>
</div>

When Choose an option... is selected, dotvvm.viewModels.root.viewModel.Form.state.HasGenerator evaluates to null. When any other option (Yes or No) is selected, dotvvm.viewModels.root.viewModel.Form.state.HasGenerator evaluates to true.

(I am using ComboBox instead of RadioButtons for accessibility reasons on mobile.)