Weird isolation issue on 2865ax WiFi only

Hi,
I've just set up a 2865ax at home to replace the ISP provided router however I've tripped over some weirdness. This is pretty much out of the box config on VDSL2, 1x 5GHz SSID, two Windows hosts.
If I plug both the windows hosts into the switch ports on ethernet cables everything works fine. NAT is fine. Internet access is fine. DNS is fine. And importantly both windows hosts can reach each other's SMB shares.
However putting either host on the WiFi, the SMB shares stop working. Only SMB is broken - everything else works fine.
I've checked:

1. Both hosts have the WiFi network set as a private network.
2. File and printer sharing is turned on properly on both hosts.
3. They're both on the same band (2.4GHz is turned off)
4. On the Draytek, isolate member is confirmed turned off.
5. Disabled the default SMB / worm firewall rule just in case.

I tried putting one host on ethernet and one on WiFi and that doesn't work either. Then swapped them around. Also doesn't work.
If anyone has any ideas I'd appreciate it! Thanks.

Are you trying to access the hosts via their computer names (e.g. \\PC1\Sharefolder) when on wifi ? If so, try using their local IP addresses instead.

Doesn’t work with IP or host name. Just hangs. Also browsing and discovery doesn’t work. 

If I shut the firewall down on both windows boxes it makes no difference. ICMP works fine.

Ok so I've tried everything. AirPrint doesn't work either from WiFi to WiFi via router. Bonjour doesn't work properly. NetBIOS doesn't work properly. I've tried every combination of VLAN, member isolation, IGMP proxy and nothing ever works. Latest firmware, no difference.

Alas that makes it useless to me so the router is being returned to the seller and I'll use the slightly molten FritzBox my ISP sent me. At least that works properly.