The bug

There are situations where the effects from a potion are negated by a beacon's effects.
As an example, a player can drink an extended Potion of Swiftness and receive the Speed (8:00) effect.
If the player then enters a beacon's radius and obtains the Speed II (0:08) effect, it overrides the potion's effect because it is stronger (as expected). However, if the player then leaves the beacon's area of effect and loses Speed II, the player does not still retain the potion's Speed effect, which I would consider the expected behavior.
In an extreme case, a player can lose 8:00 minutes of potion effect by accidentally walking near a beacon. This also applies to the Strength potion/beacon effects.