The bug

If 2 (or more) levels of the same potion effect are applied in descending order, after the higher level runs out, the GUI will continue to display the higher level with the remaining time displayed as 0:00. Despite this, the lower level’s potency is what’s actually active. The GUI will continue to display the higher level potion effect with a time of 0:00 until it disappears when the lower level of the effect runs out.

To reproduce

1. Drink a potion of the turtle master to obtain slowness 4 for 20 seconds.
2. Immediately drink a potion of slowness to obtain slowness 1 for 1 minute and 30 seconds.
3. Quickly open your inventory. Watch the duration of the slowness 4 count down.
4. When it reaches 0:00, it remains as slowness 4 with a time of 0:00 instead of changing to slowness 1 with however much time is remaining for slowness 1.