This is a known bug we inherited from dnsmasq. It is currently under active investigation by the maintainer Simon Kelley. We track this here:
https://github.com/pi-hole/FTL/issues/1166#issuecomment-920968209
where also some technical background is given. When you also check out my mail to the mailing list (link included over there), you'll see that this is very complex issue deep inside dnsmasq.
For now, you'd need to remove any custom IPv6 configuration like
server=/netflix.com/#
address=/netflix.com/::
etc. to avoid the crash from happening.
As soon as we have a bug fix available, I'll immediately set up a custom testing branch for FTL.
@Andy_Grant Just out of pure curiosity: Why do you want to block IPv6 traffic to Netflix? Is there a specific issue you are solving with this? I've seen such config lines multiple times by now (also to Spotify, etc.) from various users but I've never asked before.