?NVL( SQLResult.ProcDT, {} ) = NVL( IOL_ACT.ProcDT, {} )
.T.

Has anyone seen this behaviour? Is this a bug?

It is not a bug. If MySQL is like SQL Server, there is no such thing as an empty date. The empty date in SQL Server is NULL. NULL is not the same as empty, that is why you need to wrap it in the NVL function.