The previous behaviour of a respawn anchor was to be considered transparent by beacons and allow the beam to pass through, meaning that a beacon could be active even with a respawn anchor placed on top. This resulted in a rather nice effect of the beam seemingly coming from the portal texture of the respawn anchor. It also made sense as a portal block is transparent. 

In 1.16 Pre-2, this is no longer the case and a respawn anchor now blocks the beacon beam and disables beacons.

To reproduce: Construct a valid beacon structure, and observe the beacon is activated and the beam appears. Then, place a respawn anchor on top and observe the beacon is deactivated.