Nope, just experimented. Beacon block definitely isn't in my field of view (I've got one running up inside a tower up to the world limit), but the beacon beam is visible as long as I look at a slight angle from above. So the bug doesn't have anything to do with if the beacon block is within what the screen is momentarily showing.
Yep, I'm experiencing that one too in 1,8-pre. Looking down - beacon light there, looking up, beacon light gone.
Dolphin263: Yes, I noticed that to. Also, the closer you are to the beam, the worse it gets. I've got a couple in and around really high towers as decoration, so I could fly around them and see what happens. The bug does seem to be connected to where you are yourself in relation to the beacon block, because if you happen to be below it, you can see the beam just fine when looking up.