From what I see so far, the actual LACP functionality is working like expected. 
However, my experience of reporting on LACP ports/devices is that what is shown in the devices list is a bit uncertain, it shows wrong Link, Ch/Port and VLAN or sometimes some of them correct or partially correct.
See below explanation and figures.
I have combined port 3 and 4, on the S8 switch, for a Synology DS220+ NAS. This is configured on port 3. I did not change anything for port 4, but that should be implicit by configuration of port 3?!
There is activity on port 3 and 4 and the LACP seems to work in terms of access to the NAS, but the device name is missing:
On the devices level this is presented with the intended device name but as Link=Route10, Ch/Port=L3 and VLAN=12. Traffic shows nothing.
L3 is somewhat correct in terms of that the S8 switch is connected to that port. VLAN is erroneous, but I guess that relates to the same issue as I previously reported here:
After another try setting up the combined ports it shows Link=S8 switch, Ch/Port=4 and VLAN=50. Link reverted to blank after very short while VLAN remained correct as well as Ch/Port. Even though I would have imagined it to show 3 as it was the main port, that I configured, or 3,4 as the list of ports in the aggregation. Traffic is still showing nothing on the devices page.
Another restart of the NAS and the “device” shows up as connected to the Route10, L3 VLAN 12 again…