Hello Garth,
i did look at packages sent by OPC UA server (FactoryTalk Linx Gateway), and found that 4-byte length field of the string is 0xFFFFFFFF
Per OPC UA binary length encoding
-1 = Null string
0 = Empty string ““
greater than 0 = Regular string
This string field has a 32-bit length field of 0xFFFFFFFF, which is -1 in signed Int32 → null string.
And 0c value corresponds to string data type.
That is why ignition reads it as null value.
on this other example, it reads 0c for string data type, 0x07000000 as string field length and “testing“ string value as ASCII in each byte.
i have seen a few posts very similar to this issue, will reach out to rockwell to see if they are aware of this.