I found a problem with registrations.
I know everything about this problem and can answer all questions.

1. I run the test on spigot 1.8.8 (without Bungee)
2. Using the base SQLITE with registration 4023 players
3. there are no players on the server at the time of the test
4. AuthMe 5.6.0-beta2
5. Plugin list: AuthMe
6. Error Log: No errors
7. Configuration: default
   Descriptions of the problem:
   The problem is that players cannot always register and start playing right away.
   This crash occurs after 10-15 registrations.
   And so: the player writes / register password password
   After that, he receives two messages in the chat:
8. success: You have successfully registered!
   and at the same moment receives this message:
9. unregistered_user: The player with this name is not registered.
   And after that he sees duplicate messages:
10. register_request: Registration: / reg
    If you try to repeat the registration in the same session, the player will receive messages:
11. name_taken: A player with this nickname has already been registered.
    In authme.db, such a player does not receive the registration ip. See the screen.
    When the player re-enters the server, he will be prompted to enter a password after
    which he will be able to enter the server. But a number of such incomprehensible messages
    do not allow the player to understand that he needs to re-enter the server to enter his password.
    I hope you can help me with this.
    https://discordapp.com/channels/295623711485198357/827246583593566258/829451086514880573

There is no such problem in this version.
version: 5.6.0-SNAPSHOT-b2385

are you able to reproduce the issue without bungee and with only authme installed on the server?

Yes. Do I need to provide proof?
You can do it yourself. You need a base with 4000-5000 players. Take any nickname and try to register it. If everything worked out for you, you need to delete the registration with the command and register this nickname again. Try it 5 - 10 races and you will see the problem.

Many servers do not even realize that they are losing players who could not access the server because of this problem.

does it happen if you disable caching?

I am using the default settings.

Should I shutdown and test?
caching: false

I am testing this problem without bungeecord

You're right! When caching is turned off, the problem disappears.