Recently I checked logs from a firewall behind my SCG lives and found some drops of packet originating from a docker internal network. I don't think it should happen. Is this a known bug or something? 

The destination address does not resolve in DNS and belongs to Dell ASN  3614.

An example:

INSPECT-OUT forward: in:mgmt out:uplink, connection-state:invalid src-mac 00:50:56:a4:19:c0, proto TCP (ACK,FIN), 172.18.0.2:38100->128.221.236.246:443, len 52