I have seen this before when the server address space is not modeled correctly or returning the right type definition for some Nodes, but without access to the server it will be difficult to confirm this is the issue.
Ignition 8.1.43 includes a new implementation of system.opcua.callMethod that works around certain bugs in FreeOpcUa-based servers, and it's possible this new implementation will help here as well.
If you aren't on 8.1.43 try upgrading.