Notice the presence of the Portal noise intensifies subtitle, indicating that the sound block.portal.trigger has begun playing again even though the player isn't about to go through the portal, and in fact will exit it. This sound playing is erroneous and should be removed.

I tried testing this in 12w17a and 12w19a as I suspected this bug may have been first introduced with the client-server split in singleplayer but it didn't actually affect anything.