The WordPress back end now has a setting in Tools that shows ‘Site Health’. It tells you about any issues.
It may say you have a critical issue and should increase your PHP version to the latest version. And you thought you had because PHP version says you have 7.3 (the latest at the time of writing).
Do you have MultiPHP Manager?
Multiphp Manager was introduced by the Cpanel team with Easy Apache 4. Multiphp manager allows you to select the server default and the PHP version on a per-domain basis.
So in fact, you could have PHP 7.3 set as the server’s default PHP version but that may not be the version set for your individual domains.
To change the PHP version for the individual domains, go into cPanel and then Multiphp Manager.
Once you are in there, search for the domains and Choose the PHP version to 7.3 from the drop-down menu, and click on Apply.