Status effects change after quitting the world and back to it.

If you effect yourself with resistance level 255 using this command:"/effect give @p resistance 1000000 255"and you quit the world,after you return you will find that resistance effect level got changed to level 1.

how to reproduce:
1-start a new minecraft wolrd(a superflat world is recommended)
2-effect yourself with resistance 255 so you don't get any damage.
3-quit the world
4-back to the world
5-you'll see that resistance lvl 255 become resistance lvl 1