When you give yourself the resistance effect with the effect command and the level is bigger than 127, the resistance will work as espected, but if you leave your world, rejoin and try to take damage, the effect status displays in the GUI, but you take damage as if you would have no effect on. I gave myself the effect with a duration of 999999 (the max value), but I doubt it has to do with the said bug, even though I didn't experiment with durations as well.