Hi,
if I remember correctly, the port mirror works on the physical port, not the logical sharing port. If you want to mirror all packets of a sharing port you need to add all physical ports of the sharing.
In general you do not know which of the physical ports of a sharing are used by a given data flow, thus you need to look at all of them.
With MLAG ports, you need to mirror on both switches of the MLAG pair to see all the data.
Another caveat with a port mirror is that e.g. OnePolicy filters before the packets are mirrored, so you might not see what was sent because it was dropped before the port mirror got the packet (I encountered this problem once and at first thought the data in question was not sent at all, but I did see it when connecting the packet capture laptop directly to the sender).
Thanks,
Erik