First of all, I will mention that this bug has been reported before by a different person but ignored. MC-108980 is the same bug, but that was deemed a duplicate bug of a report from a long time ago. This, however, is mistaken. That report isn't a duplicate, and as such I need to write this one, which has more detail, to explain.

The bug is this: If you use a command to give yourself a level of a potion effect that is higher than 127, and then relog, the effect of the potion effect does not work. Levels 128, 129, 200, 254, and 255 all result in, after you relog, you being affected by the potion effect of level 0. If this were an overflow, than the potion 128 would give level 0, 129 would give level 1, etc, but it doesn't.

Furthermore, this bug is new. It only appeared in 1.11 and it's subversions. In minecraft 1.10, 1.9, and 1.8, potion levels higher than 127 work properly. As such, there is no reason why this should be ignored if it was a recent change that is causing it.

Additionally, minecraft has levels of potion effects higher than 127 do different things for other potions effects. Look at levitation for example. levels 0-127 cause you to float up, levels 128-254 cause you to float down, and level 255 causes you to float with no change in altitude. Jump boost, which is another one, was designed to work different from level 128-254, and differently again at level 255.

This bug is critical for jump boost and levitation, two potion effects which were specifically designed to work one way, and now don't.

The whole idea of high levels of potion effects, such as levitation and jump boost, is that they are invaluable for adventure maps. The fact that they worked fine in previous versions, but don't work now, means that a lot of adventure maps can not be updated or created for 1.11, as players can easily exploit situations where you are supposed to jump/float with potion effects higher than 127.

As mentioned at the start, this is a brand new bug that only showed up in 1.11 and further. Therefore this is NOT a duplicate bug report of MC-10755, which was a bug report for minecraft 1.4 Since then, minecraft has changed drastically how potions work, and that report can no longer be used to explain issues with potion levels above 127.

Thank you for reading all of this