But glue records are not supposed to be involved in Guru test, only in Sage, right?
I don't recall exactly what is being checked at each level.
And even in Guru they check xtsubasa.org's glue records, not selectel.org's as I understand it.
Since the NS records for xtsubasa.org were pointing at names outside the xtsubasa.org zone, you don't need to also specify what their IP address is. You provide the NS records for xtsubasa.org to be served by the org authoritative servers. It happens that your NS records were pointing to selectel.org names, which happen to be in the same TLD. So the org authoritative servers has NS records and glue records for selectel.org, which it could provide as additional records, when it received a query for xtsubasa.org.
As owner of the xtsubasa.org domain, you obviously aren't supposed to tell the org authoritative servers what's in the NS records for selectel.org. If you could, that would be a security problem as you could overwrite the authentic records.
And I've set both IPv4 and IPv6 addresses for selectel nameservers as glue records. Is it not how it appears to the world?
I don't recall exactly what I saw when querying the org servers for xtsubasa.org, but I definitely did not receive any AAAA glue records. And it would be the owners of the selectel.org, who would be responsible for ensuring they are there.
I checked that even Google has trouble with this
No, Google does not have trouble with this. Google has made a deliberate choice to not have their authoritative DNS servers on IPv6 yet. I don't know what is the reasoning behind that choice. I am sure once they decide to enable dual stack on their authoritative DNS servers, they won't forget to create glue records.
I changed my nameservers to ns[2|3|4].he.net now. Still no luck
You may have to wait 48 hours for caches to expire.