Cauldrons will not activate comparators if they naturally fill up to the lowest or second lowest water level

Cauldrons fill up when exposed to rain (albeit extremely slowly). Cauldrons also activate comparators based off of their water level. However, when they fill with rain, the comparator only updates when it fills to 100%; at 33% or 66% the comparator remains off unless it receives a manual update.

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Code analysis by Marcono1234 can be found in this comment.