Hi,it looks to me as this is a similar symptom to https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1000793, since it is also related to standard input.
If so, then the fix has been merged: https://gitlab.xfce.org/cquike/xfce4-genmon-plugin/-/merge_requests/1
The fix is part of the recent release 4.2.0 (https://docs.xfce.org/panel-plugins/xfce4-genmon-plugin/start#latest_release), so it would be great to get that fixed for bookworm.