Beacon state changes (e.g. activation or deactivation) aren't detected by observers. Comparators can't tell if a beacon is activated.

This goes against convention.

Steps to reproduce:

1. Place a beacon on an incomplete pyramid.
2. Use a comparator or observer to detect changes.
3. Complete the pyramid. Note that no changes are detected.
4. (Optional) Choose a status effect and click done. Still no changes detected.

I tested 32 different blocks (29 of them shown in the picture) with both a comparator and an observer, skipping the more obvious or redundant blocks.

Blocks with observer-detectable changes: furnaces, pistons, redstone ore*, farmland, campfires (lighting/extinguishing), end portal frames, walls, powered rails, note blocks*, daylight detectors (inversion & daylight change), stairs (shape change), trapdoors, cauldrons, lamps, shulker boxes, bells (when activated through redstone), flower pots, brewing stands (inserting or removing a bottle), rails (angle change), water, and respawn anchors. Continued, with blocks with no visual changes: Tripwires, saplings (not fully tested), jukeboxes (unless you count the "Now Playing" display), and leaves (log distance/presence).

Several notable blocks with comparator-readable states:
Respawn anchors, end portal frames, containers, jukeboxes, cauldrons, and composters.

Blocks with undetectable state changes:
Beacons, conduits, and dragon heads.

They have block state (for lack of a better word) and visual changes caused by player interaction and (for the first two) environmental variables. As a significant element to the game, the fact that beacon block states aren't registered or recognized is an anomaly.

Extra notes:
Rosebushes were not categorized because although you can interact with them, their block states can't be changed, as opposed to, for example, a composter. I didn't fully test cacti or conduits since observers break them, and water breaks comparators. For the conduit testing setup, the observer was placed last and didn't detect a change while it disabled the conduit. Through the F3 overlay, I found that it's unlikely that those block states could be detected even without the problems.

I'm aware that I may be using "block state" for slightly different meanings. I hope my point is understood clearly enough. Perhaps the real bug here is that the block changes aren't classified properly, or that they aren't classified in such a way that they would show up in the F3 menu like all blocks with observer/comparator detectable changes.