Is it possible to send the hint from OPNsense itself?
Yes, to me it sounds like you’re already getting a big enough prefix from your ISP (all devices getting a /64), but you’ll have to request a bigger prefix from OPNsense. I believe it should give you the options to do this when you set the IPv6 mode to DHCPv6 on OPNsense, but I can’t say if your ISP router will handle it.
I believe I’ve actually had this happen with actual VLC, I think I just hit pause and then play and it was fixed. So maybe pause it for half a second after your seek.