I think the stealth detection is intended
It's WAI, but not for the reason you think it is. It's included to prevent stealthers from skipping and/or dancing around bosses like they have been apt to do in *every* other MMO. It's also not as complex and "stealth is worse than non-stealth" as most of the people posting here make it out to be: when you get in range of the boss, you get the "red reticule of warning" on your face which provides you with ~5 seconds to either run away or fight the boss. If you can't be arsed to pay attention or make a decision that quickly, well, maybe a stealth class isn't for you. If you start a boss fight because everyone else was out of stealth and you were not, try dropping stealth before passing by bosses.
It's obviously not a bug since there's an explicit mechanic for it so, rather than complaining about it because "omg, I can't just run around in stealth all the time and dance next to bosses while doing so" realize that, just *maybe*, you're doing it wrong and maybe you should think about doing it right.