The bug

Unlike all other GUIs, you can open the advancements overview while traveling to the nether or back.

In singleplayer, the game is paused then while in multiplayer, the game continues to run, causing the advancements overview to be closed as soon as you've changed the dimension.

How to reproduce

1. Enter a portal
2. Press the "Advancements overview" key (default: L)

Code analysis

In class EntityPlayerSP, in function onLivingUpdate, it is defined that a screen's doesGuiPauseGame function must return false if it should not be able to be shown.

If that function is implemented into GuiScreenAdvancements like so:

public boolean doesGuiPauseGame()
{
    return false;
}

the overview can not be invoked while the player is inside of a portal. However, this also causes the overview to disappear when you open it from the pause menu while standing inside of a portal; and also the game does not stop while being in the advancements overview, which probably is not intended.