Peripheral doe not terminate according to spec

I'm writing test code to verify the softdevice operation according to the spec.

The goes like this:

 The central sends the feature exchange command, when the feature exchange response is received, the central sends a terminate command with a NACK response. 

The expected response is the peripheral sould terminate the connection and not send another exchange response.

The actual response is the peripheral sends another feature exchange response after receiving the terminate command.

Any ideas?