I have some Blue Lake Stringless pole beans in my garden. I built an 8 foot
tall trellis for them to climb on. They very quickly shot up to the top, and
now I have 4+ foot of vines waving in the air above the trellis, for a total
length of ~12+ feet. Is this normal? How do you normally grow pole beans?

Doesn't sound like a problem to me :-).

What I do is put up a trellis/cage/fence (usually closer to 4 feet
tall although I guess some of mine are closer to 8), and when they get
to the top of it, just let them flop over (in other words, do
nothing). At least for me, they don't really wave in the air so much
as grow sideways or fall down, so that might be a slight difference.

If they really stay 12 feet in the air once they have fruit, you'll
have a hard time picking them.