Would be pretty easy to do a fix, actually. Just check to see if the target has 0 threat and is in stealth, if so, ignore that target as being the second aggro target. Depending on how they implemented the second target aggro stuff, it might take as little as 5 lines of code.
Knowing how lazy Blizzard programmers are though (and I do know, I've ripped Starcraft apart many timesover the years), it's probably something that is not so easy to fix. It's probably a continual check, which patching it how I suggested would introduce huge numbers of extra CPU cycles to process, which isn't a good fix at all.