Exactly! I've just figure out that as well. There is Record File Format Sample Rate in cubase project settings , which has to correspond to that of audio interface and KPA. Originally my audios were in 44.1 kHz , that's why the S/PDIF clock kept changing by cubase. After switching to 48 kHz and reformatting whole my recordings - the latter problem has gone!
But this does not make clear the original problem - why S/PDIF Output in KPA keeps switching by itself with 44.1 kHz.
Nonetheless, it is nice to have a workaround versus not having at all. Thanks.