The bug

Instant effects (namely instant health, instant damage, and saturation) are intended to be applied, well, instantly – for a single tick. They have an immediate effect then disappear, not even noticeable on the effects pane. This works fine for drinkable/splash potions, etc.

However, when applied with /effect, the time is treated as seconds, causing the effect to be applied for 20 times longer than intended. This makes it impossible to apply an instant effect for the intended amount of time, causing instant damage to be always fatal and instant health to fully heal you, regardless of the amplifier selected.

Note that treating time as seconds is intended for non-instant effects.

How to reproduce

In survival mode, run the following command:

/effect @a instant_damage 1 0

Expected Behavior: You instantly take three hearts of damage (because amplifier 0 was specified).
Actual Behavior: You take four separate hits of three hearts and die.