Hi all, thanks for the feedback. The problem is fixed though I can't say precisely how.
We were under some pressure to get it resolved quickly so I tried just setting up a whole new server with the latest Ubuntu, hoping that some difference in the server environment would help
First I tried pairing the existing CentOS 7 web server with Ubuntu for the database but that didn't help. So I tried making the new Ubuntu system the web server pointing to the existing CentOS 7 MariaDB server and that fixed it.
I wish I could share what difference between the two actually solved it, but I am just glad to have it fixed. I did go straight to PHP 7 on the new web server so that could have been a factor, but hard to say without further trial and error. And I'm not too keen on that now that it's working.