We believe we've also observed this bug at Google on Debian 11. We confirmed that packets are received by the kernel, but dhclient doesn't catch them. The root cause appears to be a race condition, and we provided some details in https://gitlab.isc.org/isc-projects/dhcp/-/issues/264 to ISC, but we understand dhclient is being deprecated.
The bug was also reported to Ubuntu at https://bugs.launchpad.net/ubuntu/+source/isc-dhcp/+bug/1926139 and there's a detailed discussion of the bug, reproduction cases, and proposed fixes, the last of which was accepted into Ubuntu Jammy. We built a test package with Ubuntu's fix applied to Debian 11 and confirmed over the weekend (more than 100k runs) that it resolves the issue. Because of some risk that the fix could cause issues elsewhere, the patch included an option to revert the change at runtime. I haven't found evidence of anyone needing to use that option. Oddly we also find that Debian 12 does not have this issue, even though the later version of dhclient does not include any fix for this. There must be some other reason why the race condition is not hit there.
