Crappy ones also sometimes leak UDP packets. Or all DNS queries or whatever. If you use crappy VPNs it's your fault if you then don't get the protection you want, no matter the transport protocol.
Or rather: Using IPv4 doesn't guarantee non-crapyness of a VPN provider.
But: Working IPv6 support guarantees at least some level of proficiency by the VPN provider, so they might be more reliable candidates to begin with.
Crappy VPN services do all sorts of crappy stuff.
But there's more needed with IPv6 than routing properly. The VPN provider needs to assign IPv6 addresses to customers, and that's harder than just NATing stuff. It's almost like being an IPv6 ISP.
But I've done a toy implementation. To get "anonymous" IPv6 addresses, so I could test VPN service clients for IPv6 leaks, without pwning myself. I needed a little help from an IVPN engineer, but it wasn't that hard.
But crappy ones don't. And some people end up using crappy ones, because they don't know any better :(