Cubase digital coaxial signal (SPDIF)problem

My audio interface is RME ADI2 PRO,macOS Big Sur V11.6.1
In Cubase digital recording mode, the input of VST CONNECT is set to SPDIF, and the monitor of recording track is turned on, and regular signal noise will be generated.
However, my digital interface is not connected to any device。
I changed the same LOGIC Settings, but it is very normal. So I’m sure there’s nothing wrong with the audio interface
The screenshot below is the signal I recorded.
Please help me

截屏2022-03-01 下午3.50.58