I am pretty sure that only "travel" should be played when exiting the nether portal.
How to reproduce: Go to a nether portal, whait to cross, and once you are teleported in the other dimension, both sounds come in play : block.portal.trigger AND block.portal.travel
And that sounds wrong. 😛
How to fix: making the portal not playing block.portal.trigger after crossing it.
"block.portal.trigger" is for just standing in the portal, so this is intentional.