Issue with default generated ula_prefix - IPv6 internet not working

I have IPv6 (Native, with PD) enabled via my ISP and have been testing it over the last few weeks.

It had been fine on a separately compiled version of LEDE by one of our community members, but after someone posted in another forum about having an issue getting the LEDE snapshot (from late May) to work, I decided to do some testing.

To give some quick info;

  • Compiled vanilla (no custom modified packages) from latest snapshot, other than CPU overclock patch and uci-defaults file I added.
  • IPv6 upon first boot allocates fine, including prefix delegation to the WAN6 interface
  • IPv6 address assigned to LAN (both the local one and a delegated one from ISP)
  • IPv6 and IPv4 allocate fine to all clients (windows, Mac, iOS, Linux)
  • No IPv6 internet is available.

I have narrowed it down to the ula_prefix set within the /etc/network file - this is system generated each time the router is reset to factory settings as far as I know.
If I set the prefix back to what I had before I started tinkering or generate a new one via a website I found online, then reboot the router, all IPv6 connectivity seems to return and work perfectly.

Has anyone else had this issue? Can anyone point me in the right direction to troubleshoot further? I'm not super clued up on IPv6 and I think I may have tunnel vision!

In case it's potentially a package I haven't installed, here is my current installed packages list: