Seems like the issue affects RU region mostly. Google Public DNS is making no-ECS requests towards our DNS servers located in Russia (it looks like the source is in Finland according to geo bases). Probably the issue is not related to our implementation. Any thoughts? Thanks in advance.
I can confirm that our resolvers in Finland represent most of the ones that have been failing to auto-detect ECS support from your name servers.
Here are numbers of resolvers that are detecting ECS for each of your anycast IP addresses:
group F (80 jobs) .100: 17 .101: 34 .200: 22 .201: 23
group R (110 jobs) .100: 9 .101: 10 .200: 11 .201: 6
group G (110 jobs) .100: 6 .101: 8 .200: 2 .201: 8
group M (110 jobs) .100: 1 .101: 4 .200: 4 .201: 8
The few resolvers that do detect ECS are getting a strong signal, and are sending all of their queries with ECS.
Unfortunately, the queries to your authoritative name servers that are sent with ECS are anonymized in our logs in a way that removes your responses, so I can't tell how or why your name servers are failing to respond with ECS (and therefore causing our resolvers to stop sending ECS).
I wonder whether your name servers are returning truncated responses, either because of large response data, or because of rate limiting.
If you can capture some traffic and check, that might be a clue.