@Edrikk When I used your cron job I get an error in the System Log:
Thu Feb 24 13:27:00 2022 cron.err crond[26439]: USER root pid 26644 cmd arp-scan -qxlN -I br-lan | awk '{print $1}' | xargs fping -q -c1
Nevermind, I see here that all Busybox messages are written with an error code. I guess that doesn't mean there's an actual error.