IPv6 issues for wired devices with Intel ethernet - Fibre

Disclaimer - This may not be a 2Degrees / Chorus Fibre issue, but keen to hear thoughts.

For as long as I can remember I have had IPv6 issues, and have finally had some time to track down the actual issue.

Ipv6 has always worked fine on iOS and Android devices, the problem was on Windows 10 PCs and laptops (a mixture of Lenovos and a homebuilt ASUS based PC)

The symptoms were that when IPv6 was enabled, some websites would kind of work, but then time out. For example, in GMail it would log in using Chrome, but when I clicked on my labels it would just hang, and then an offline message would show. In Edge, I couldn't even login. On other sites, downloads would start, and then stop. Disabling IPv6 on the device or on the router would resolve the issue. Long term I have left IPv6 enabled on the router and just disabled on the affected devices.

From further testing I have worked out it is only an issue when connected via ethernet (all devices were Intel ethernet and a variation of the I219-V). Running Wireshark on the ethernet would show a lot of duplicate ACKS, so something possibly wasn't right.

Anyway, I finally stumbled across this that described my issue:https://forums.intel.com/s/question/0D50P00004UK5SrSAL/help-intel-i219v-tcp-checksum-offload-ipv6-issue?language=en_US

Changing TCP Checksum Offload (IPv6) to [Disable] or [Tx Enabled] on the ethernet adapter resolves the issue. The interesting thing is in the above Intel forum post, it links to:https://downloadcenter.intel.com/download/27263/Disabling-TCP-IPv6-Checksum-Offload-Capability-with-Intel-1-10-GbE-Controllers

And this states:"In some situations where a client machine is connected via some specific Optical Network Terminals (ONTs), and data is appended after the packet checksum, the network adapter can drop receive packets when using TCP-IPv6 Checksum Offload for receive traffic."

Could this be an ONT issue? Has anyone else had similar issues with wired devices? In terms of the ONT our install was in 2012. Current router is the standard 2Degress Fritzbox 7490.

Cheers,Dan