Since you have also posted about the FP4030MR HMI, is it possible that the value is being written to DM[1036] from the HMI?
Or if you have another device or software transmitting data via Modbus, could that be the reason?
It is also possible that your program is looping through DM[]s with a variable index and you would not be able to directly search for DM[1036] in this case. Please check your FOR and WHILE loops that reference DM[]s in case the index could reach DM[1036].
If not, please email your program to support@triplc.com