After some more digging I was finally able to discover how do I get in my LAN those weird IPv6 prefixes from.
It turned out that my Nest Thermostat is rarely sending IPv6 RAs, offering some IPv6 ULA prefixes, causing my devices to use Nest Thermostat’s ipv6 link-local address as their default IPv6 gateway where Nest is definitely not a router. I found that all Nest devices like Smoke detectors etc are doing the same thing.
This combination makes my iPhone XS basically not operational when connected via MikroTik AP set to 802.11ac. It is working fine when MikroTik AP is set to 802.11an. I believe due to some unknown bug in processing 802.11ac packets, MikroTik is preventing the iPhone XS (and most likely the new 2018 iPad Pros as somebody reported) from fallback to IPv4. Maybe it is dropping IPv6 packets, that I don’t have time to check.
All my other devices like iPad Pro 10.5” (2017), iPhone X, MacBook Pro (2017), MacBook 12 (2017), iMac 5K (2015), PC with Windows 10 don’t have problems with connectivity even with those silly IPv6 addresses and gateway assigned. Only the iPhone XS and only on the MikroTik’s AP configured to 802.11ac. I have other Asus 802.11ac AP in the same bridged network and when I’m in its range my iPhone XS works perfectly fine, even tough I still have those IPv6 addresses assigned from Nest Thermostat.
As a temporary workaround I have connected my Nest Thermostat to an isolated Guest SSID on my Asus router, so it is no longer advertising those IPv6 addresses to my home network.
Immediately I got rid of this crazy problem and can enjoy my 866Mbps connection via MikroTik’s AP!!!
To MikroTik staff : If you want to reproduce this issue I suggest you connect some Nest device to your network as the only device that advertise IPv6 prefixes and connect your iPhone XS to it using 802.11ac.
You have to wait couple minutes because Nest sends RAs every few minutes or so.
If you have other routers sending IPv6 advertisements you may still not be able to reproduce this issue, so make sure Nest is the only source of IPv6 in your LAN. Your main internet connectivity still has to be over IPv4. Make sure you’re using FQDNs in Safari when testing connection. If you don’t have any Nest equipmnent you might try to connect additional router to your LAN which will only send IPv6 RA’s and ULA prefixes but will have forwarding disabled - sort of dead end IPv6 router.
Flowers and bottles of bourbon are very welcome, if you wanna send one PM me for an address
Btw. This problem still existed on latest iOS 12.1.1 and on latest RouterOS 6.44beta40