TCP-RTU Modbus Behaviour

I am experiencing inconsistent behavior with the Node-RED Modbus nodes when communicating with the same Modbus device.

When connecting through a TCP-RTU gateway:
Only the “Modbus-Read” node works
The “Modbus-Getter” node, which works perfectly when only reading the TCP, no longer works over TCP-RTU
The error message is: "Error: Timed out"
The warning: "Modbus Failure On State connected Get More About It By Logging"

TCP: Getter works, Flex-Getter fails
TCP-RTU: *Only Read works, Getter and Flex-Getter fail

How is that possible?