When a player is trying to cross trapdoor in a 2 block tunnel and one of the trapdoors blocking the tunnel is open then the player is able to activate swimming action to go through the door.
When a player is trying to cross trapdoor in a 2 block tunnel and one of the trapdoors blocking the tunnel is open then the player is able to activate swimming action to go through the door.
This is normal, opening the trapdoor activates crawling. See MC-147612, and the comment by @unknown in MCPE-175077.