Ipv4 and ipv6 only work on the Internet because of constant maintenance by many people working in many different organisations. Ipv4, being effectively mandatory, gets most of that attention. Ipv6, being a nice-to-have future- proofing option, gets less. And so you are far more likely to encounter issues, in the general internet, where connectivity is not working properly, and even if you have the energy to debug it, you are likely to find the problem is not on your end and the only option is to fall back to ipv4 and wait for it to be fixed.