I'm still wondering if it has to do with what version of php you're running. I'm running 5.4.23.

It doesn't happen to everyone. It seems quite random, which makes me suspect that it only manifests itself when certain conditions are met.

The Stovebolt Geek
https://www.stovebolt.com/ubbthreads/ubbthreads.php

Server Information
UBB.threads Version 7.7.5
Release 20201027
Server OS Linux
Server Load 0.16
Web Server Apache/2.4.6
PHP Version 7.4.33
MYSQL Version 5.7.43
Database Size 1.85 GB