When you perform a double transfer, the server resource packs are unloaded.

Here are the steps I have noticed:
1. Join a server that has a required resource pack
— > Connection state: PLAY
— > In-game, with the resource pack applied

2. Be transferred to a server with /transfer.
— > Connection state: CONFIGURATION
— > Loading screen, with the resource pack active

3. Be transferred again from the configuration phase.
— > The resource pack is unloaded. (The problem)
— > Once the transfer is complete, in-game, without the resource pack.

When I connect to a server that has a required resource pack and transfer to another server (1 transfer), I keep the resource pack. If I do a second transfer right after the first (before PLAY state), the resource pack is unloaded.

Maybe it is a security but I have not found any information on this subject.