Is it possible for `sudo` to fail temporarily with the correct password? Hacking suspected

Question

TL;DR: sudo with the correct password failed repeatedly in my home server but it suddenly started to success after a few minutes. Can this behavior be explained without assuming the existence of an attacker who has the root privilege?

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This morning, I read The xz package has been backdoored - Arch Linux News, which strongly advises the users who use xz 5.6.0-1 or xz 5.6.1-1 to upgrade their systems.

After that,

1. I opened my laptop to connect to my Arch Linux server in my home, which is also publicly accessible via a private key, via SSH and executed sudo pacman -Sy to prepare for upgrading my system.

2. I was prompted the user's password so I typed the correct one. However, all of the three attempts failed:

   $ sudo pacman -Sy
   [sudo] password for user:
   Sorry, try again.
   [sudo] password for user:
   Sorry, try again.
   [sudo] password for user:
   sudo: 3 incorrect password attempts
   

3. Then, just in case, I configured my router to disable port forwaring, making my server inaccessible outside LAN. (Only I can connect to the server now.)

4. After that, I repeatedly tried sudo many times while confirming I didn't enable CAPS_LOCK etc. I also tried to connect to the server from iPhone and execute sudo. All of the attempts failed. This was EXTREMELY strange as I connect to the server everyday; the possibility where I typed incorrect passwords for every attempt is very low.

5. A few minutes later, sudo suddenly started to success (with the same password).

Can this behavior be explained without assuming the existence of an attacker who has the root privilege?