How to Troubleshoot Writeback Failures

We have a FileMaker database with corresponding fields in MariaDB. When records are created in SQL, they are sync'd with FileMaker and the record is indeed created and the writeback field has an auto-incremented value, but the writeback field is not always "written back" to SQL. What is worse, if the record is modified in SQL, the null value in the writeback field is sync'd back to FileMaker so that a field which is defined as Indexed, Auto-enter Serial, Always Validate, Unique actually winds up with a null value!

Any idea how to troubleshoot this problem. We can recreate it quite frequently.