The bug

When you set an effect in a beacon that requires more than 1 layer, and then break the layers so that it shouldn’t be able to give the effect anymore, it will still give you the effect.

How to reproduce

1. Build a 2 layers tall beacon pyramid
2. Set the resistance effect or the jump boost effect
3. Break one block of the lowest layer
   It will still give you the effect even though the pyramid only has one full layer.

Code analysis

Code analysis by marcono1234 can be found in this comment.

Original Description:
I can create a four-tier pyramid, then set the beacon to provide the primary power of the strength boost and the secondary power of regeneration, and then break the fourth, third, and second levels of the pyramid, and the primary power of strength will continue to refresh, although only out to 20 blocks, and the secondary power will not refresh.

If I then repair the pyramid's second layer, the range of the primary power will extend out to 30 blocks. Repairing the third layer will increase the range to 40 blocks. And repairing the fourth layer will return the beacon to full functionality. If I break the first level(leaving the beacon block itself intact), then the primary power will stop, but if I then restore only the first level of the pyramid, the beacon will continue to provide the primary power of strength out to 20 blocks.

Ultimately, the beacon is properly checking to see if it should apply the secondary power, and how far a range it should apply primary or secondary powers out to, but is not checking to see if the TYPE of primary power it is set to provide is still supported by the appropriate number of layers required to initially select it.