I have never seen such a problem myself, but have you tried restarting the MySQL server?
And just out of curiousity, do you use InnoDB as the engine or the default MyISAM (or anything else)?
Just default MAMP SQL install.
I did solve the roblem though -----
I think I have found a very unexpected feature in ESS
If you import records into an ESS MySQL T.O.
ANY field in a row is "skipped'" because of a data type mismatch
That record is treated by FMP as "modified by another user while you were editing it." from that point on and NOTHING* will allow you to change that and the record (any field) is thenceforth un-editable in FM except by overriding this dialog (doing that lets you save a new value but does not fix the record)
*Not even shutting down and restarting SQL and restarting FM
That sounds like a bug to me...
Anyway, glad to hear that you solved the problem and thank you for providing the details!
It is worth sharing.