Validate restriction in Azure AD B2C custom policy

I have the following ClaimType in a custom policy to test out collecting user timezone during sign up:

  <ClaimType Id="extension_timezone">
    <DisplayName>Timezone</DisplayName>
    <DataType>string</DataType>
    <UserHelpText>Enter your timezone</UserHelpText>
    <UserInputType>DropdownSingleSelect</UserInputType>
    <Restriction>
      <Enumeration Text="(UTC-08:00) Pacific Time (US &amp; Canada)" Value="(UTC-08:00) Pacific Time (US &amp; Canada)" SelectByDefault="false" />
      <Enumeration Text="(UTC-06:00) Central Time (US &amp; Canada)" Value="(UTC-06:00) Central Time (US &amp; Canada)" SelectByDefault="false" />
      <Enumeration Text="(UTC+00:00) Dublin, Edinburgh, Lisbon, London" Value="(UTC+00:00) Dublin, Edinburgh, Lisbon, London" SelectByDefault="true" />
      <Enumeration Text="(UTC+12:00) Auckland, Wellington" Value="(UTC+12:00) Auckland, Wellington" SelectByDefault="false" />
    </Restriction>
  </ClaimType>  

This displays correctly in the sign up flow but I have noticed that the input is not validated against the restriction. Steps:

1. Select item from drop-down
2. Use browser tools (e.g. Chrome DevTools) to change the value of the selected option
3. Submit the form

I would expect there to be validation to check that the submitted value matches one of the enumeration entries, but this does not happen. I can include extension_timezone in the output claims and see that the value reflects the changes I made that do not match any of the enumeration options.