I did an upgrade on a WRT1900acs v2 router (my backup router) that was running 18.06.4 (I think) via LuCI using the image "openwrt-19.07.0-rc1-mvebu-cortexa9-linksys_wrt1900acs-squashfs-sysupgrade.bin" keeping the old configuration. Everything seemed good except the clients could not connect or ping each other. The router could reach/ping the clients. This was the case whether or not "Isolate Clients" is set in the LuCI Wireless -> Edit -> Advanced Settings.
I noticed that if I connect an ethernet wire to one of the clients (a Raspberry Pi) and hook it to the router that I could connect/ping the Pi using either the IP associated with the ethernet connection or the IP associated with the WiFi connection from another client. When I disconnect the ethernet on the Pi I can no longer connect to the Pi's wireless IP
I decided to reinstall 19.07.0-rc1 to see where the isolated client behavior starts. After I reinstalled 19.07.0-rc1 I restored the configuration using LuCI System -> Backup / Flash Firmware -> Restore Backup. After it rebooted it was working, clients were not isolated. I installed a few packages that I wanted, here's the list:
Everything still working. I changed the 5 Ghz wifi from WPA2-SPK to WPA3-SAE. Everything still working. I changed the 2.4 Ghz wifi from WPA2-SPK to WPA2-SPK/WPA3-SAE mixed. This is when it stopped working. I don't know if this last step actually caused the problem since setting both wifi networks back to WPA2-SPK and rebooting (and unplugging for 5 minutes) does not fix the problem, clients are still isolated. Pretty much all clients are on the 2.4 Ghz network.
Here are a couple bits of diagnostic which may or may not be helpful:
-------------> when everything is working (clients not isolated):