As mentioned let’s start on the OSMC box that is your server/client. So enable debug logging on your OSMC machine that you have configured to update the database first time.
Sure, all machines that use Kodi and that share a database need that configuration. That your OSMC machine also acts as a MYSQL server doesn’t mean that Kodi directly uses it.
Actually I guess that is your simple issue, you sofar only have configured it on your PC. Means that when after you configured it on your PC it started to scrap into you Database. Now if you configure it on OSMC it will directly use the database.
Just a reminder depending on your file sharing you might also have to look into path substitution.