Redmine checks, if the value is available for the custom field, as follows:

if custom_value.value.present?

But for multiple custom fields (e.g., lists, that allow selecting multiple values), the original @nil@ value is replaced with @[nil]@ (not sure where exactly). So, @[nil].empty?@ returns @false@ and the custom field gets rendered.

This issue affects 3.0.x and 1.4.x versions of Redmine.

P.S. Can be solved by montkey-patching CustomFieldValue as follows:

alias_method_chain :value, :extended

...

def value_with_extended
    value = value_without_extended
    if custom_field.multiple? && value.present? && value.size == 1 && value.first.nil?
        nil
    else
        value
    end
end