Just to report, with RT-AC88U as main router and Blue Cave as mesh node, things are working flawlessly since a week on the latest official (Non-Beta) firmware. No devices have been kicked out or the network has not vanished or any sort of funny business. It has given reception to areas which were dead zones like toilets. I use ethernet onboarding by connecting LAN port of RT-AC88U and WAN port of Blue Cave. Both the routers were updated to latest Aug firmware and set up after performing factory reset. No wifi settings were touched and Blue Cave was added to mesh node from ASUS Router app.
The only caveat is that this Asus Mesh does not assign optimal connection speeds quickly. For example, when my Google Pixel 2 will be in front of RT-AC88U, it will show 833 Mbps connected speed. Then if I carry it to other room where it is in front of Blue Cave, it will still remain connected to AC88U and show a connection speed of 90 Mbps. Now ideally I expect it to connect to Blue Cave but it doesn't. Only rarely if I am lucky, it will connect to Blue Cave at full 833 Mbps.
I also tried to turn off it's wifi and on turning it back on, it still connected to AC88U situated far away with 90 Mbps even with Blue Cave in front of it.
If this way ASUS Mesh is supposed to work and not as per what I expected, then this whole mesh thing is pure horse crap.