When converting from Vanilla 1.17 to Spigot, a player cannot enter an End gateway portal from the main island.  The player returns immediately to the source portal as if there is no destination.

This has been confirmed when converting a 1.16.5 Paper Build 775 world to 1.17 as well as a new 1.17 vanilla created world to Spigot 1.17.

To reproduce, create a new world in vanilla, visit the end and go through an end gateway from the main island.  Return to the main island and shut down your world.  Load that world with Spigot 1.17 with no plugins or datapacks, let it convert and load into the world.  Try to go through the end gateway and it will fail.