No it's not intended, it's a bug. The OP isn't complaining about the bosses being able to see stealthed players, he's complaining about the range of their vision. A stealthed character can be spotted from a LOT further away than a non-stealthed one - that shouldn't happen.
Bit of a stretch calling it a bug, it is what it is.
As for the OP, use your brains, you know how it works now, just don't use stealth near the champ bosses. I worked this out back in JAN, and never had a problem since.