I would think it has something to do with this caveat of Gateway Mode:
When Mesh is enabled in gateway mode, and when the WAN IP address is obtained via PPPoE, the Master AP cannot be part of a Mesh tree. However, Mesh can still be enabled and any member AP can be a Root AP or Mesh AP.
But I was able to use the Master AP as the Root AP of the mesh, so it must not be the case that the WAN IP address is obtained via PPPoE. I've been testing without connecting the master AP to my ISP, so I'm not entirely sure if that's relevant.