Should (vanilla 1.21.5 behaviour): When a filled composter or cauldron is pushed behind the block a comparator is reading from, the comparator turns on. (Though the same is not true for a composter or cauldron being moved away from that position...)
Reproduction steps: Build the "contraption" provided in the screenshot, placing the redstone block last. Before placing the redstone block, use any compostable item to fill the composter to at least lvl 1.
Is: The comparator does not turn on, see attached screenshot.
Impact: Deviates from vanilla redstone behaviour. Breaks redstone designs that rely on this feature, primarily the chromoss style bonemeal farms that are currently popular with more technical players.
1.21.4: This has been a non-issue before our server updated from 1.21.4 to 1.21.5 Spigot, but I did not verify with a basic 1.21.4 Spigot installation.
Please classify as mayor if the impact seems big enough.