Perhaps it's detecting a Mesh Firmware update is needed too? I just updated to 386.7_2 and was seeing the same behavior, so I checked my Mesh device and it's telling me there's a firmware update for it too.
After updating the Mesh device, and then going back into the Firmware Update page so...
+1 to using the Ethernet Backhaul method. AC68U as main (386.3_2) and AC68P as node (386.3_beta3 = RT-AC68U_9.0.0.4_386_55919-g4b63ee0).
What isn't explicitly called out anywhere here is: When doing the initial mesh node discovery, WAIT for the node's Wi-Fi connection icon to change to an...