Some more details on this , in case it might affect any other users, or possibly might assist with guest network issues more generally...
Further testing here with my AC86U has shown that, with both Merlin 384.19 and Asus-WRT 3.0.0.4.386.41634 firmware, I can add guest networks to either the first or the second column and I can see other clients in the guest network. However, with Merlin 386.1, this only works if I add guest networks in the first column. If add a guest network to the second column with 386.1, guest client isolation is always present; i.e. I cannot see any other clients. AND, contrary to what I'd previously thought, this does not seem to depend on the 'Set AP isolated' setting in the Wireless -> Professional tab. So something seems to be a bit messed up here.
As mentioned earlier, I've been testing this on the 5GHz network only, as I don't have any guest clients on the 2.4GHz network.
I've left the router running 386.1 with the guest networks set up in the first column; fortunately I've not seeing any untoward log messages (yet) and all seems to be working OK.
And just to clarify my testing method, I'm using the Fing app on an Android mobile phone to scan for other devices, plus I'm also checking with the Google Home app to see whether any Chromecast devices are available.