Don't worry. Someone will know the answer to this.
In the meantime here is a little food for thought / questionable workarounds:
If you have enough routers you could attach individual routers to the mesh nodes by LAN (or AP/STA? - more demanding on the mesh nodes resources though) and have REAL LANS (run them as routers not dumb APs) per se on independent subnets. Individual routers running 19.07.7 should be able to isolate clients. This is what I am moving towards until pioneers such as yourself carve a path forward.
Another alternative is to make individual VLANS for each device you want isolated. Do laugh. I have done this before just for a laugh with a different router / product family. This is more work - but UCI is arguably cut and paste after the first few are set up - you can just copy them and change a few variables. Layer 2 isolation is imperfect even on other products and may not be secure. Thus just separate everything onto isolated VLANs. Even that is not perfect and does not guarantee security. The number of VLANs you can make is memory constrained. Make a note of how much memory they are using each time you create one.
Turn off software/hardware flow offloading on all routers - maybe it is bypassing layer 2 functionality the same way it impacts QOS/SQM?
The problem your routers are exhibiting is probably a mixture of router hardware and router software not working correctly more so than your config IMHO.
It looks like BATMAN and OpenWrt are not playing nicely on layer 2 (unable to isolate clients) despite most of your settings looking good. If this is the case then this is something the developers would have to address.
Maybe your phone is caching results (like firefox caching webpages) try verifying isolation by alternative means. Erase the app and reinstall?
Buy more used routers cheap off the internet? That's what I am doing. It's fun!
Solution #1 costs more but then you have more redundancy and functionality / distibuted processing power.
Solution #2 doesn't cost anything. VLANS are free! If you can make one, you can make a 1001!
Last thoughts. IPC40xx product family has always been problematic with VLANS and many choose to avoid it and go for mediatek for VLAN or other chipsets.
I have IPQ4019 / EA8300 meshed with regular 802.11s (you have IPQ4018 in the ASUS I think) so I am probably not going to have success with layer 2 isolation either unless the developers can patch them or whatever may be the case. Eventually I will test and configure BATMAN. But I am fairly happy with the ease of use and stability I have right now with plain 802.11s and my config is twice as fast as my 150Mbps internet connection.