The IPS sensor trigger for a custom signature with severity low

Hi everyone!

I set custom IPS signature on FGT 7.0.2:

config ips custom
edit "TCP.SYN.Rate.Limit"
set signature "F-SBID( --attack_id 7313; --name TCP.SYN.Rate.Limit; --protocol tcp; --tcp_flags S+; --flow from_client; --rate 3,60,limit; --track src_ip; )"
set severity low
set location server
set comment "An attempt to prevent brute-force attacks to a TCP server."
next
end

but it's triggering the IPS-Default sensor which is set as follows:
config ips sensor
edit "IPS-default"
set comment "Prevent critical attacks."
config entries
edit 1
set severity medium high critical
next
end
next
end

By checking IPS Event Logs can see that the unit is erroneusly processing the custom IPS signature as "Critical".
Intrusion Prevention
Profile Name IPS-default
Attack Name TCP.SYN.Rate.Limit
Attack ID 7313
Incident Serial No. 112889839
Direction outgoing
Severity critical
Message custom: TCP.SYN.Rate.Limit

Why IPS sensor trigger for a custom signature with severity set low?

Thank you!