Hi, thanks for the report.
I also experience both IPv6 addresses, randomly, only with Firefox under Windows.
Note: a request to ipv4.ipleak.net (having only A record, not AAAA) is received server-side as IPv6, inexplicably. It seems related to some network 'optimization'.
Currently, I disabled the keep-alive server-side, it seems related and the issue is resolved, but still under investigation.
We also have an 2nd problem: DNS detection throws errors sometimes, but this is not related to the issue above.