Check with those who got their black ONT (aka HG8240) replaced with the white ONT (aka HG8240HS), did M1 took back the former?
Curious, exchange for what reason? Who initiated the exchange?
Until recently, I wasn't able to get IPv6 working for many many years.
Tried ASUS and LinkSys routers both didn't work. Shifted house and migrated the line over with the same black ONT - still didn't work.
Anyway, M1 support knows nuts about IPv6 - each CSO will say different things. One will say IPv6 not supported, another will say it should just work, i.e. it's not their problem, it's your equipment problem.
Long story short, switched to using Google Nest Wifi Router in my new home - and M1 support actually redirected my call to Google Support.
Google collected the router logs and analyzed and confirmed that it is M1 problem. Sent M1 email to say that Google support says it is M1 problem and miraculously IPv6 worked automatically soon after - till now I've no idea what M1 did at their backend.