I spent many many days and weeks with very very specific and random problem.
Put data like this:
Then create PHP PDO connection and simple query like into test.php:
Create cron like this:
I believe that after some time you will see that php fetchAll() or fetch() will return empty result.
After I replaced DB to MySQL 5.7 than result will ALWAYS return requested row.
I think that some combination broken MariaDB stability, because my VPS server has enough memory, enough cpu capacity, enough disk space. Other services working well, no other problem and soo on. PHP is from Remi repository for example 7.4 version. MariaDB repo is original mariadb. I tested with all, but my main test was on 10.5.9 version. Tested on CentOS Linux release 7.9.2009 (Core). Core Linux 3.10.0-1160.21.1.el7.x86_64 (SMP).