The bug

Credits only appear when you have left the end at least one time respectively when the seenCredits tag of a player is already true (1b).

How to reproduce

1. Create a new world
2. Go to the end
3. Beat the dragon for the first time
4. Go through the portal back to the Overworld
   → The credits are not shown
5. Go to the end again
6. Go through the portal back to the Overworld
   → Now the credits are shown

Code analysis

Based on a decompiled version of 17w13a

It looks like the values for the SPacketChangeGameState packet (deobfuscated name: id) are just interchanged when the method EntityPlayerMP.changeDimension(int) (deobfuscated name: nn.b(int)) creates it. It should use 1.0f when the player has not seen the credits yet.