[Ignore]-property loses value after IIS reset

I have a Block type that allows nested blocks for structuring block layouts.

[ContentType(DisplayName = "Row Layout Block")]
public class RowLayoutBlock : BlockData
{
    [Display(Name = "Blocks")]
    [AllowedTypes(typeof(IRowLayoutItemBlock))]
    public virtual ContentArea Items { get; set; }
}

IRowLayoutItemBlock contains a flag that tells the front-end that this block should be handled a little bit differently.

public interface IRowLayoutItemBlock : IContentData
{
    bool IsRowLayoutItem { get; set; }
}

I don't want this flag to be visible/editable in the CMS editor and I've added the [Ignore] attribute on the inherited member.

[ContentType(DisplayName = "Image Block")]
public class ImageBlock : BlockData, IRowLayoutItemBlock
{
    [Ignore]
    public virtual bool IsRowLayoutItem { get; set; }
}

To set this flag I've subscribed an event handler on SavingContent and CreatingContent

context.ConfigurationComplete += delegate
{
    var contentEvents = context.StructureMap().GetInstance<IContentEvents>();
    contentEvents.SavingContent += (sender, args) => RowLayoutBlockContentEventListener.SetIsRowLayoutItemOnItems(args);
    contentEvents.CreatingContent += (sender, args) => RowLayoutBlockContentEventListener.SetIsRowLayoutItemOnItems(args);
};

public class RowLayoutBlockContentEventListener
{
    public static void SetIsRowLayoutItemOnItems(ContentEventArgs args)
    {
        if(args.Content is RowLayoutBlock rowLayoutBlock && rowLayoutBlock.Items?.Items != null)
        {
            var contentRepository = ServiceLocator.Current.GetInstance<IContentRepository>();
            foreach (var item in rowLayoutBlock.Items.Items)
            {
                if (contentRepository.TryGet<IRowLayoutItemBlock>(item.ContentLink, out var rowLayoutItem))
                {
                    rowLayoutItem.IsRowLayoutItem = true;
                }
            }
        }
    }
}

This all work perfectly until I do a restart of IIS when all the flags are unset/set to false for some reason.

Is there a better way to do this or am I just missing something?