my isp has situation that my ipv4 behind nat and ipv6 port is open. is it still works? looks like it doesnt

AFAIK NAT with ip4 networking is still prevalent with ip6 running along side

Tixati seems fine with both stacks and I have not seen to many problems

Now if only there were more seeds

this is a situation often called CGNAT (Carrier Grade NAT)

and I have never heard of an ISP doing customer specific port forwards though it's possible in theory.. (not that the average ISP CSR knows their head from their A$$)

so... your client will be unconnectable on IPv4, you might notice reduced speeds or you might notice when you are trying to leech something with no/few other connectable seeds (BT requires at least one connectable peer in a swarm)

really... using a VPN that supports port forwards is probably the best solution here, Tixati has good support for binding to a VPN adapter directly.

I have seen many class A subnets on ipv4 so in principle tracking torrents by trolls is hard as hell

Mask is 255.0.0.0

so something like 10.x.y.z is a giant NAT network

Smaller class B and class C subnets are also widely used