I am running PHP 7.4.29 (my core files have been modified) as well as MariaDB 10.5.15 so I am thinking that it is not a MariaDB issue.
I cannot imagine Redis touching the vbulletin DB directly
Looks like vbulletin 4 does not support Redis?
https://vborg.vbsupport.ru/showthread.php?t=323530
Have you tried to clear the system cache?
Have you tried to disable all mods and see if the issue continues?