Check your versions against what is supported: Which ODBC data sources/drivers are supported with External SQL Data Sources? | FileMaker
The connector seems to be right one, what about the version of MySQL itself?
I had a similar issue. Try adding a 5s pause after the initial refresh. If this helps start trimming the pause down to the shortest time for it to be reliable 100% of the time. It is also good in my experience to have a refresh after the set field as well.
MySQL version is 5.6.36. So it is earlier than the one listed as compatible with FM (5.7.14).
I doubt I will be able to have the webhost upgrade the MySQL server. Also this was not a problem with FMS 15, nor does it appear to be a problem client-side in the hosted database.