Description of problem:
A PPPoE connection with ipv6.method=auto should work as follows:
1. IPV6CP sets up the interface identifier and default route.
2. We client send an IPv6 router solicitation.
3. The peer sends an IPv6 router advertisement.
4. Depending on the advertisement, we do SLAAC or DHCPv6 to configure an interface address and get a delegated prefix from the peer.
5. Interfaces with ipv6.method=shared are configured using the prefix which was delegated to us by the peer (if any).

This worked under NetworkManager-1.34.0-0.3.el8.x86_64.rpm, but breaks under the next version of the package (NetworkManager-1.36.0-0.1.el8.x86_64.rpm).  NetworkManager 1.36.0 does not send the router solicitation, so the process stalls and we never get a working IPv6 connection.

Version-Release number of selected component (if applicable):
NetworkManager-1.36.0-0.1.el8.x86_64.rpm

How reproducible:
Always

Steps to Reproduce:
1. Configure a PPPoE connection with ipv6.method=auto, connecting to an IPv6 capable ISP.
2. Bring up the connection - no IPv6 address will be assigned and no interfaces with ipv6.method=shared will have addresses assigned.
3. The PPPoE traffic itself can be tcpdumped, which confirms that no solicitation is sent.

Actual results:
No IPv6 address assigned.

Expected results:
IPv6 address should be assigned.

Additional info:
The interface's /proc/sys/net/ipv6/conf/*/accept_ra is set to zero both under 1.34.0 (working) and 1.36.0 (broken).  I have read that NetworkManager intentionally sets this to zero and handles router solicitation itself rather than leaving it to the kernel, so the accept_ra setting is probably a red herring.