If you are on windows you can force windows to use IPv4 for your netconnection somewhere in the network settings given your provider and router supports IPv4 and IPv6. But this is only required if you plan to host a masternode because Masternodes work with IPv4 only. I would not recommend it.
I'm pretty sure I saw a couple of masternodes on the list with ipv6 at some point. But it might have been another coin.
Is the ipv6 not working a bitsend thing only or for masternodes in general?
You are correct. I confused it with DASH. I self run a BitSend Masternode on IPv6 a year ago.
However from my experience it doesn't work proper on IPv6 and you are missing out on many payments.
At least so it was for me. You will not see that at first because it shows running and you get paymets but just not as many as you should. Those IPv6 masternodes become often dropped out by the other IPv4 masternodes.
If you want to avoid troubles you should stick with IPv4 for Masternodes.