RT-AC86U reporting:
1st attempt to upgrade from 384.11_2 to 384.12 failed. Dunno why it failed. The progress bar still continued until 100% but the message below the progress bar, there was a long paragraph which said it has failed. After 100%, the login screen came out, after login, the firmware was still 384.11_2 and everything worked normally.
Tried to upgrade using the same firmware file again, the message below the progress bar showed success. The progress bar continued until 100% then login screen came out. After login, the firmware shown as 384.12.
I waited for 5 minutes then hard reset the router (I always do this for each time of firmware upgrade). Now 384.12 is up and running for 12 hours. Everything is fine.
I remember somebody also with RT-AC86U posted here earlier also encountered the same problem of first attempt failed but second attempt success. Other than this issue, everything is OK.