When a Beacon is below the player's level, let's say 10 blocks below the player, and there is a hole so the beacon emits the beam, but if the player looks at it from an angle, the beam won't show up. I also tested it on multiple maps, multiple graphical options, other Minecraft versions but the problem occurs still.
On the picture, what you can see is the angle i was watching the beam when it appeared, and disappeared, the other beacon in the background though is on my ground level and it's beam wont disappear at any angle, however the one that is underground disappears from a specific angle, I tried it with colored beams, it's the same result.