Solved:

    public class DataConsentFixed : Umbraco.Forms.Core.Providers.FieldTypes.DataConsent
{
    private static readonly string s_trueString = true.ToString().ToLowerInvariant();

    public override IEnumerable<string> ValidateField(Form form, Field field, IEnumerable<object> postedValues, HttpContextBase context, IFormStorage formStorage)
    {
        Dictionary<Guid, string> fieldValues = form.AllFields.ToDictionary((Field f) => f.Id, (Field f) => string.Join(", ", f.Values ?? new List<object>()));

        List<string> list = new List<string>();
        if (field.Mandatory && !postedValues.Any((object x) => string.Equals(x.ToString(), s_trueString, StringComparison.InvariantCultureIgnoreCase)))
        {
            bool flag = true;
            if (field.HasCondition())
            {
                flag = !field.Condition.IsCircular(form) && field.Condition.IsVisible(form, fieldValues);
            }

            if(flag)
            {
                string item = ((!string.IsNullOrWhiteSpace(field.RequiredErrorMessage)) ? field.RequiredErrorMessage : "Consent is required to store and process the data in this form.");
                list.Add(item);
            }
        }

        return list;
    }
}

In the Validate Field method of the DataConsent field type the visibility of the field was not taken into account. Overriding the method with some code taken from the FieldType.cs did the trick.