That's strange. I've been trying to see if there's a cleaner way by invoking screen in an /opt/etc/init.d/S99rtrmon-screen kind of script, to avoid a kludgey "sleep 30", but the variables still seem to require explicit definition.
Bash:
#!/bin/sh
PROCS=screen
ARGS="-dmS rtrmon /jffs/scripts/rtrmon.sh -monitor"
PREARGS=
PRECMD="export SCREENDIR=/root/.screen HOME=/root"
POSTCMD=
DESC=$PROCS
ENABLED=yes
. /opt/etc/init.d/rc.func
I don't know how this worked or didn't work on the previous versions of Entware and screen. I don't see many obvious changes to the variable handling in GNU screen to explain it specific to this tool.