Hi,
The opcodes for Generic Power OnOff models are wrong. They weren't changed when the model was copied from the Default Transition Time server Model.
/** Generic Power OnOff model message opcodes. */
typedef enum
{
GENERIC_PONOFF_OPCODE_GET = 0x820D,
GENERIC_PONOFF_OPCODE_SET = 0x820E,
GENERIC_PONOFF_OPCODE_SET_UNACKNOWLEDGED = 0x820F,
GENERIC_PONOFF_OPCODE_STATUS = 0x8210
} generic_ponoff_opcode_t;
From the Model Specification

I'm using NRF5 SDK For Mesh v3.1.0.