Hi,
We had the same problem trying to send the recipe file to a press.
For us it was fixed in 8.1.15
" 5480: The function system.opcua.callMethod incorrectly encodes ByteString parameters**
Support coercion of primitive arrays to ByteString when writing to tags or calling methods with ByteString type input arguments."
"Nightly 8.1 Changelogs - 2022 - #15 by sreis"
give it a try
Andrea