Tcpdump work on openwrt

openwrt18.06(monitor) :

tcpdump -i wlan0 "ether[0xa:4]==0x13223344"

You can capture the packets after filtering the mac work well.

tcpdump -i wlan0 "ether[0xa:4]==0x13223344"
Unable to capture packets after adding mac filter.It work bad.

But when I do not fill in the mac address filter, save all the captured data to a file, and then filter the file, I can capture the data packets

tcpdump -i wlan0 -w /tmp/test.pcap

tcpdump -r //tmp/test.pcap "ether[0xa:4]==0x13223344"

Filtering from a file it seems to work fine.

what could be causing this ?