Here's every mention of ubbt_myid in the code:

Code
/usr/local/www/html/ubbthreads/admin/loginas.php:$html -> ubbt_setcookie("{$config['COOKIE_PREFIX']}ubbt_myid","$Uid",time()+$config['COOKIE_LIFETIME'],"$COOKIE_PATH");
/usr/local/www/html/ubbthreads/libs/html.inc.php:			$this->ubbt_setcookie("{$config['COOKIE_PREFIX']}ubbt_myid","{$user['USER_ID']}",time()+$config['COOKIE_LIFETIME']);
/usr/local/www/html/ubbthreads/libs/html.inc.php:			$this->ubbt_setcookie("{$config['COOKIE_PREFIX']}ubbt_myid","{$user['USER_ID']}","0");
/usr/local/www/html/ubbthreads/libs/ubbthreads.inc.php:$myinfo['id'] = get_input($config['COOKIE_PREFIX']."ubbt_myid", "cookie", "int");
/usr/local/www/html/ubbthreads/libs/user.inc.php:			$html->ubbt_setcookie("{$config['COOKIE_PREFIX']}ubbt_myid","0");
/usr/local/www/html/ubbthreads/scripts/logout.inc.php:	$html -> ubbt_setcookie("{$config['COOKIE_PREFIX']}ubbt_myid","0", time() - 31968000);

Looks to me like it might be getting set to 0 in two places; libs/user.inc.php and scripts/logout.inc.php. I'd have to look at the code closer to see what the "0" refers to though. Maybe one of the whiz kids knows off the top of their head?

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