I'm currently using 3004.388.5 firmware. I tried several times to update the 3004.388.6_0 firmware via the router's web interface. After completing the update process and manually rebooting the router, the firmware number did not change and remained 3004.388.5.
I have the RT-AX88U Pro and successfully upgraded to 3004.388.6 today fine, so I can confirm it can work. I was going from the .4 release, so slightly different.