Euclid would be turning in his grave.

I am aware that some of the transitions are very visible, however this is most of the time caused by insufficient processing power. The way it works is a plugin runs on a spigot server that has some areas which if the player enters, they get teleported to another similar location but has a different layout that would not be possible with regular vanilla Minecraft.

While a lot of this is done with command blocks teleporting the player a set relative distance, the plugin is necessary for rotating the player around a certain point. This was used mostly in the area with crimson nylium on the floor. You can turn a corner and find you are back where you started by rotating the player's location around a certain point. Rotating the player means if you're looking at the north face of a block (eg nether brick block), you could then be looking at the west face of the block after the rotation.

Blocks have slightly different lighting in Minecraft depending on which side you are looking at, even if the light levels are the same. This is because Minecraft uses it to help the player be able to distinguish different sides of a block. This is especially important for concrete, for example, which is almost entirely block colour. Therefore when changing which face of the block you're looking at, the lighting changes subtly too. This change can be quite noticeable if there is little else to focus on.

Another problem is that when rotating the player, unless the blocks on the floor are rotationally symmetrical, it becomes painfully obvious when a rotation takes place.

The solution to these problems could be to create 4 parallel worlds for each transition, and have each world in each transition have it's own teleportation transition to the other parallel worlds in the other rotation transitions. This would become increasingly time consuming and very tedious, so I stuck with the server-plugin implementation.
Locally this may be better for smoothness of transition, but that does also mean areas with wooden planks in the floors have their own parallel rotations to ensure this the 4 parallel worlds are implemented fully. The lines in the planks' texture which might normally go along a corridor would then cut across it. Even if there is no jump between them, going to one place then coming back a slightly different route to find the textures are offset by 90 degrees is not preferable to having slight jumps in textures due to lag or not quite rotationally symmetrical textures, in my opinion.