When you block damage with Unbreakable the shield is supposed to last longer, in fact the grenade cooldown/timer acts normally. However it is bugged and after an amount of time the unbreakable shield is released early despite still blocking damage. The Youtube video demonstrates.
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Hey there, Thank you for reporting. We're aware of this issue and are investigating. According to other reports, this bug happen only when you have Shackle grenade equip. The aspect should be working as expecting without the grenade equipped.