I don't know if it's just my game or not, but it seems like leaves from destroyed trees don't decay if they are touching a standing tree. Leaves from trees that don't touch any other decay fine though.
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Indeed, as long as it's within a certain radius of a log, it won't decay, even if the log is of a different type. Note that 1.13 also changed the radius; it used to be 4 blocks but now is 6 blocks (since 18w22a, due to MC-1809)
I'm pretty sure that as long are in a certain radius of logs, leaves don't decay.