OpenWrt IPv6 issue?

I have an odd problem with IPv6. It appears to be common to OpenWrt 18.06.8 and 19.07.4, and Inteno IOPSYS which is OpenWrt based.

On a freshly powered-up router, I can run ping -6 on my laptop to remote destinations the other side of the router with no problems.
However, if I physically disconnect the WAN Ethernet cable so the interface drops, then reconnect, the WAN6 interface comes up (eventually) but IPv6 behaves oddly. Ping -6 to the remote destinations works for a while (roughly a minute or so), then stops working with destination unreachable error messages for a minute or so, then starts working again, then not, with no regular operation/no-operation pattern or time interval. Web browsers that try to use IPv6 have problems.
Restarting the WAN6 interface does not resolve the problem, but power-cycling the router does.
On OpenWrt 18.06.8 collectd ping doesn't parse IPv6 addresses, but collectd ping does work for IPv6 addresses on 19.07.4: and pinging from the router shows the same behaviour - it is not a PC artefact, or local networking issue. Using the statistics package to graph the IPv6 pings shows a packet loss rate of about 45%.
Is this something anyone else has experienced/resolved? Any ideas what it might be? I have tried three different routers (TP-Link Archer C7 v2, TP-Link Archer C7 v4, Inteno something or other), and have the same symptoms, so it isn't the router hardware.
My next step is to bypass the router and simply put a Linux PC directly attached to the WAN Ethernet, but I don't know when I can try that. If anyone recognises this problem and knows the resolution, please reply!

Also check if any those helps or not:

ifup wan6; ifup lan
/etc/init.d/odhcpd restart

Compare before and after the issue happens:

ifstatus wan6; ifstatus lan
ip -6 address show; ip -6 route show table all; \
ip -6 rule show; ip6tables-save

This sounds similar to this bug:

It seems to affect many ath79 devices making them, well, less useful than they should be.... but as far as I can tell, no work has been done to fix it. I think it is the ethernet driver.

If this is the issue here then you can work around it by configuring a lan port for wan use, but then you loose a port.

1 Like