DefaultValue not working

Hello everyone

In a Block a [DefaultValue] is not working. It is an existing Block, existing properties [DefaultValue] is working fine. I added a new property and it won't set the default value. Am I missing something?

This is the new property within the Block

[Display(Order = 20, GroupName = "Image d'arrière-plan", Name = "Safezone", Description = "La grandeur en px ou % de la safezone à partir de la droite")]
[DefaultValue("33%")]
public virtual string SafezoneImage { get; set; }