Sorry about the long delay in answering to this. I apologize.
There is a VPN provider specific option on each VPN type on Sailfish OS 4.3.0 ->. This was a bit problematic to implement similarly for each VPN type as they do tend to have different requirements each.
If you disable IPv6 support / enable IPv6 leak protection on the advanced settings pages the feature to prevent DNS leaks to IPv6 networks is enabled for the duration the VPN is connected. Unfortunately this has to be done for each VPN you are using.
And as of now, only OpenConnect does support both IPv4 and IPv6, all others support IPv4 only. This restriction comes from the ConnMan plugins.
But please try the feature and let us know how it works. All information regarding this is valuable.