Sorry for the late reply but I just added a second AP and had the same problem. Here's a more efficient fping recipe which runs faster and doesn't blow the arp cache:
arp-scan -qxlN -I br-lan | awk '{print $1}' | xargs fping -q -c1
I added a cron job to run this every 15 minutes. Note that as of 19.07.5, arp-scan 1.9.5 is broken; you'll need to install arp-scan 1.9.7 from snapshot (https://downloads.openwrt.org/snapshots/packages/$OPENWRT_ARCH/packages/arp-scan_1.9.7-1_$OPENWRT_ARCH.ipk).