Hi, small update on this bug. This effect actually only happens when killing epic mobs (in epic dungeons) or on some foundry mobs. If killing normal mobs or normal dungeon mobs it seems to work. This randomness seem to confir that it it indeed a bug and not a willing feature
Same prob (well i can only have 1 stack of the buff instead of 3) and other TR 4set bonuses are also broken (ex master duelist cooldown reduction does not proc anymore)