I think that was reported along with many skills that broke when leveled to 4 - other skills were fixed - this one not.
Tenacious Concealment is supposed to prevent stealth loss from damage on levels 1-3 and gives additional stealth regeneration on rank 4. Unfortunately leveling it to 4 breaks the stealth loss prevention completely - you lose stealth after taking damage as if you didnt have the passive at all.
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