I dont know if this is intentional or a bug, but to me seems buggy behaviour.

I am diagnosing EDNS by using the following command. Which makes a test server send responses to show the EDNS size used.

‘dig +short rs.dns-oarc.net txt’

on unbound and bind 9.9 This will result in large packets of over 4000 bytes.
It also reports a EDNS buffer size of 4096.

On bind 9.10 the first request has packets below 512 bytes and reports and EDNS buffer size of 512.  However if U run another query shortly after it then reports larger sizes of over 4000 bytes. So it seems it needs multiple requests to use large EDNS packets.  I have confirmed this behaviour on 3 different servers all of which run FreeBSD.  The EDNS size seems to be stored in some kind of cache that expires because eventually a request will then drop back to a 512 byte limit again.

Result of first query using bind 9.10

rst.x487.rs.dns-oarc.net.
rst.x499.x487.rs.dns-oarc.net.
rst.x457.x499.x487.rs.dns-oarc.net.
"2001:41d0:1:a16c::10:1 DNS reply size limit is at least 499"
"2001:41d0:1:a16c::10:1 sent EDNS buffer size 512"

Result of second query using bind 9.10

rst.x4090.rs.dns-oarc.net.
rst.x4060.x4090.rs.dns-oarc.net.
rst.x4066.x4060.x4090.rs.dns-oarc.net.
"2001:41d0:1:a16c::10:1 sent EDNS buffer size 4096"
"Tested at 2017-08-19 22:56:40 UTC"
"2001:41d0:1:a16c::10:1 DNS reply size limit is at least 4090"

Result of any query made on unbound or bind 9.9

rst.x4090.rs.dns-oarc.net.
rst.x4060.x4090.rs.dns-oarc.net.
rst.x4066.x4060.x4090.rs.dns-oarc.net.
"2a01:4f8:201:5465::2 DNS reply size limit is at least 4090"
"2a01:4f8:201:5465::2 sent EDNS buffer size 4096"
"Tested at 2017-08-19 23:03:20 UTC"

Please let me know if you need more information.

regards

Chris