About dropper lines not working as they should, the ticks are varying depending on the position in the game chunks and the simultaneous activation of other redstone components, causing the non-functioning and order switching, breaking item lifts, transport fast and other machines that need the dropper order to work consistently.

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For replicate: method 1
1. Make a line of 3 droppers vertically
2. Place a block in front of the first dropper, put redstone dust on top of the block
3. Put item in the first dropper of the line
4. Energise the redstone dust

*Observation:*Works normally, the item will pass through the 3 droppers and will come out

5. Now place a "redstone clock", using 2 observers facing each other, anywhere in the same chunk as the droppers

Expected result: the item will pass through the 3 droppers and will come out.

Result: the item get stuck and not leave the dropper

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Another easy way to replicate
1. Do steps 1 to 3 (method 1)
2. Place an observer facing the restone or the droppers in the same chunk
3. Energize redstone dust

Expected result: the item will pass through the 3 droppers and will come out.

Result: the item get stuck and not leave the dropper

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Another way to replicate
1. Do steps 1 to 3 (method 1)
2. duplicate the same system, or connect redstone dust to other droppers in the same chunk
3. energize the two together

Expected result: the item will pass through the 3 droppers and will come out.

Result: the item get stuck and not leave the dropper

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Very important!

Don't forget that the bug works only in the same chunk, so follow the steps and reproduce in the same chunk, otherwise it will work as expected.