ipv6 on my Motorola mt8733 is broken

Other than checking on their website, I have not. The next step would be to send an email to customer support. Until recently ipv6 was working just fine, and the settings that are available to me have not changed. I'll give that email a try and report back.

Thank you I look forward to hearing back from you. I will also follow up after the weekend to check in if I do not hear back from you. 

I received a reply from Motorola customer support. They requested the serial number and CM MAC address which I have supplied. They also suggested I perform a factory reset. It didn't correct the problem, which I have reported. I await their next reply.

Thanks for the update. Please keep us posted. 

It's finally fixed! Tracked down to a flaw (feature?) of motosync, an app used to access the router settings using a cell phone. I'll quote the Motorola rep below:

I've done some checking with my colleagues.  They reminded me that there is an issue with Motosync that forces the router to work with IPv4 on the LAN side.  So, being onboarded to Motosync will cause IPv6 addresses to not work on the LAN side.  There was a suggestion to completely reset the MT8733 on both the router and server side, and then do not add it back to Motosync.  That should cure the issue.

The rep went on to offer to remove my motosync profile on the server side, so it wouldn't be reloaded after performing a factory reset on my side. It didn't appear to work immediately, but after I had restored my WiFi and other settings, ipv6 was back to working on the LAN side. I also took the further step of removing Motosync from my phone and will only use the web interface from now on.