If you do not have a AAAA record present it is most likely automatically generating a record to the IPv4 server.
Either that or it forces the IPv4 connection on the device attempting the IPv6 connection.
I have only ever experienced issues with having the correct server value set for the IPv4 record and (accidentally) had the IPv6 point to the old server I was using, causing a whole host of issues on certain devices. But, I just deleted the IPv6 record and 24 hours later the change had propagated and it was resolved (I now don't have an IPv6 record declared - but it works swimmingly).
This is what I have experienced with other host sites (not squarespace).