You can enable network wide client isolation feature on any one of the WLAN this will prevent the communication between clients connected in 2 different wlan’s/vlan’s.
This feature is available under WLAN configuration.
Ping might not be the best test as it is ICMP traffic and maybe not treated the same way as TCP/UDP - I don’t know, it’s just a guess. Can you actually make connections to devices in the other VLAN?
Either way, it would seem that whichever device is acting as the router on your network may be allowing the traffic to flow between VLANs, so have you tried configuring rules there to prevent this? For example, the router on my network does not block inter-VLAN traffic by default, if it has a route, it will use it unless it is set not to. Even then, this doesn’t apply to pings unless a specific firewall rule is set to block ICMP too (which is why I suggested it).
From my understanding, the network-wide client isolation setting prevents hosts on the same SSID from seeing each other - is that the case for you with it enabled?
Can you please let us know what is the firmware version that you are using on the AP and also share the techsupport file of the AP to my email id ashok.kumar@cambiumnetworks.com