My setup until 81049 was:
RT-AC86U (AiMesh router)
RT-AC68U (AiMesh node)
I updated both to 81039 and didn't notice any issues. Then 81049 was released. I performed the update like I always do: via the firmware upgrade section in the web UI. My RT-AC86U updated fine, but when I logged back in to the web UI, my 68U was nowhere to be seen. I looked at it and saw that the power light was rapidly flashing. I made several attempts at resetting it (e.g. holding reset while powering on; holding WPS while powering on, holding both reset and WPS while powering on, etc.) and none worked. My 68U seems to be bricked.
i have the same setup as yours exactly and it's working fine. i think when the power led is flashing fast it means it's in rescue mode. did you try the firmware restoration tool?