Cyclopropinon They accidentally made it so that the detection range is rounded down to the nearest even number. For example, when mcMMO says you have to move at least 3 blocks from the old fishing spot, you actually only have to move 2 blocks. I found it while reading the mcMMO source code.
(And unrelated to the range bug checkbox: You, the player, don't have to move at all. Only the bobber needs to land in a different spot next time you catch a fish.)