(Tested for Vanilla 1.21.1)
The time it takes for a comparator to activate/light up after an item has been placed inside of a hopper minecart it is looking at is random. From my non-exhaustive tests, I have seen a time range from 3 to 22 ticks pass between the time the item is placed inside and the comparator lights up. The time it takes is not location-based.
Steps to reproduce:
1) put up some minecarts on detector rails (in vanilla minecraft, only detector rails work, as far as I know that's intended behaviour)
2) place comparators that are observing the contents of the minecarts
3) input the command /tick freeze
4) put an item into each of the minecarts
5) repeat the command "/tick step 1t" until they light up, this way it's obvious that they light up at different times
6) Repeat to see that it's not based on location
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Can not reproduce in 1.21.5
They probably didn’t place the container minecarts in the same gametick. This behavior is not random. Activator rail sends comparator every 10gt’s since turned ON.
You must sync minecart placement, for the comparators to sync up as well
Can confirm.