IPv6 has no fragmentation, so MTU hurts more

IPv4 lets routers fragment; IPv6 moves that responsibility to the source and drops oversized packets with an ICMPv6 Packet Too Big.

Many firewalls block ICMPv6 along with ICMP, so the source never learns, PMTUD fails, and the symptom is "small packets work, large ones hang".

ping -6 -M do -s 1452 example.com

Work down from 1452. The 40-byte IPv6 header plus 8 bytes of ICMPv6 leaves less payload than IPv4.

Fix the interface MTU to 1480 or 1400, or allow ICMPv6 type 2. Blocking ICMPv6 is the most common self-inflicted wound in IPv6 networks.