automatic shutdown despite active client connection

I'm using a HTPC running libreelec 8.2.1 with vdr backend. I'm watching TV via a raspberry3 with vnsi. Everything is working just fine. Now I tried to enable power saving and configured the server to shut down after 15 minutes of inactivity. The server will happily wake up for planned recordings and the client will wake up the server as well via WOL. But if the server is idle and I'm watching TV from the raspberry, the server still will shutdown after 15 minutes (it does not matter whether I'm watching live TV or some recording). There must be some mechanism for vdr to tell kodi to NOT shut down as recordings during absence are working just fine. But this mechanism seems to be missing for remote clients. Does anybody know how vdr is telling kodi that it is active and that the system should not be shut down automatically?