ThemableCheckboxSelectMultiple does not support switchable/switched behavior

Horizon form supports the switchable/switched mechanism which allows to show some field depending on another field.

When MultipleChoiceField is 'switched' field and the widget ThemableCheckboxSelectMultiple is used, the field is never hidden.
There is no such example in the horizon repo, but we have an example in neutron-vpnaas-dashboard [1].

'subnets' field ("Local System Subnets") should be shown only when "subnet" is selected in 'type' field, but 'subnets' field is always shown.

If I replace ThemableCheckboxSelectMultiple with CheckboxSelectMultiple (Django default widget for MultipleChoiceField), the field is hidden/shown conditionally as expected. Thus I believe it is a bug of ThemableCheckboxSelectMultiple.

[1] https://github.com/openstack/neutron-vpnaas-dashboard/blob/1705fc586bf88f4ac1c296185995a6cbc7c7edbc/neutron_vpnaas_dashboard/dashboards/project/vpn/workflows.py#L147-L156