The slime block/sticky piston combo is very buggy, at last count it took 20+ clicks of a button to get the desired boost. This has only been an issue in 1.16, as it never happened in 1.15 or any other time. The actual redstone mechanics are working as they should be, only the boost up has been screwing up.
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We're tracking this issue as MCPE-62419, so this ticket is being resolved and linked as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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