I discovered a weird interaction with the mod Immersive Portals. Not a blocking issue, it does not prevent from using any of both mods features, and working around it is easy. But it can be annoying, and may be used as a cheat vector (non-allowed teleportation).

Expected Behaviour
While being in an elevator cabin, being able to walk normally, getting in and out, while starring at its reflection in a Immersive Portals enabled mirror surface.

Actual Behaviour
The player moving speed is heavily reduced. The player reflection moves normally, at normal speed.
Getting out of the elevator cabin teleports instantly the player at its reflection position.

Reproduction Steps
Steps to reproduce the behavior:

0. Install MTR and Immersive Portals
1. Build an elevator and spawn an elevator cabin. A single floor is needed.
2. Build a glass wall parallel to a side of the elevator cabin, and light it up with a flint and stone to spawn a mirror portal.
3. Walk into the elevator cabin
4. Watch your reflection into the mirror.
5. Walk in the cabin while keeping the mirror visible on screen. -> The reflection will start to move out of sync.
6. Walk out of the cabin. -> You may be teleported according to the reflection position.

Crash Report
This issue does not causes a game crash.

Screenshots

In this first screenshot, the situation is normal : I am in the elevator cabin looking at myself in the mirror.

After a few steps, my reflection walked away way faster than my real body.

Environment

• Minecraft Version: 1.19.2
• Fabric Loader: 0.14.14
• Installed Mods:
  • MTR 1.19.2-3.2.0
  • Immersive Portals 2.3.1

Additional Information
I suppose that this is related to how the player movements are handled by the mod inside moving vehicles. These handling is certainly not applied correctly "in the mirror". By the way, the issue concerns probably the trains and others MTR vehicles too.

I post this report in MTR's issue tracker because that very way of handling people movement of MTR could be, I think, hiding the cause of this issue. But it is just an intuition, it may be a bug in Immersive Portals' end;

Also, I wonder if this could be used as a cheat vector. But it would be very cumbersome way, since the reflection walks normally in the "reflected" world, at normal speed while the player is slowed down, before being teleported in the effective position of the reflection. It could be used to teleport in a position where the player should not be, but I have no test case for that.