On VirtualBox 6.1.14, with the exact same pfSenseVM, but one of the interfaces changed from 82540EM to virtio, I still see the configured IPv6 address used as the source OSPF6 address, so changed the vNIC type didn't change the behavior.
However, the funny thing is that I then added two new vNICs to the VM (one 82540EM and one virtio) and then configured an IPv6 address and OSPF6 on both. Lo and behold, both new interfaces send out OSPF6 Hellos with the link-local address!
I compared the interface and frrospf6dinterfaces>config sections for those interfaces and they're the same as the interface that use the configured IPv6 address. I really can't explain the behavior in difference that I'm observing.