The other way would be to let clients use default VLAN (i.e. de-activate corresponding entries in /interface wireless access-list) and see if inter-client communication starts to work fine.
Your theory is right:
Pings:
Host1 - 2.4GHz <-> Host2 - 2.4GHz on VID 30 "Native" - Ok
Host1 - 5GHz <-> Host2 - 5GHz on VID 30 "Native" - Ok, but not right away. ~9 pings "unreachable"
Host1 - 2.4GHz <-> Host2 - 2.4GHz on VID 20 "ACL" - Fault
Host1 - 5GHz <-> Host2 - 5GHz on VID 20 "ACL" - Fault
Host1 - 5GHz <-> Host2 - 2.4GHz on VID 20 "ACL" - Fault - Even more strange 'cause I thought it should work via software bridge "LAN Bridge"
So, is it a bug or a feature? =) Is this bug already known? Maybe it was corrected in modern ROS vers?
According to the manual On QCA8337 and Atheros8327 switch chips, a default vlan-header=leave-as-is property should be used.
But this is for switch-chip. Don't think it will depend. But checking now...
Checked. Nothing happens on Wi-Fi.