Some kind of Path MTU Discovery failure. Usually ICMP is blocked, but just as common is that the ICMP packet comes from an incorrect (ISP internal) IP, and then gets dropped. This can happen due to NAT-ing or MPLS, or such. This used to be much more common 20 years ago, but it definitely still happens.

OpenVPN has a mtu-test option that can sometimes help with this, I don't know if Wireguard has something similar.