It sounds like the problem is with the DSE unit. The READ_VAR uses the PLC CPU and the IO scanner uses the processor of the NOE card so these two ways of using modbus is completely different, if one has a bug the other one most likely will not have the same bug. NOE cards have firmware updates that might solve different kinds of strange problems but this will not be solved but something like that I think. I would test with different kinds of modbus PC tools to see if the problem is easily reproduced and then show the manufacturer if it is. A free program I can recommend for this is ComTestPro.