I got the same problem here in my country, Using FTTH from Viettel. I noted them with the issue that some sites I can ping but can’t access via IPV6. But their answer kind of claim their customer is dumb to work thing out.
At the begining, I asked them to bridge my mikrotik but it take them a week to do that, and the tech come to my house is not familiar with mikrotik things, but I heard somewhere that the ISP has already import huge amount of mikrotik. I request speak to higher tech and this person claims that mikrotik can’t work well with pppoe or IPTV. But guess what, I figured all thing out by follow most intrustion from here. The more I’m working with the tech guy, the more I found that those tech guys, even at higher level and we call them NOC don’t know much about their job either. It took me couple months to figure about pmtu here, and another couple months to figure this clamp-to-pmtu rule. Thanks to the right keyword.
I know this topic is old enough but there still people like me have difficult time with the ISP like this one and they don’t even want to help at all. Thanks to sb56637 and mducharme.