This has been specified in 1997 by PGP 5 for a good reason. We talked often enough about this and it does not help to repeat your ideas over and over again. RFC9580 specifies a different protocol than OpenPGP as specified by RFC2440 and RFC4880 but alas grabbed the name OpenPGP for this.

b) For non-confirmed keys it returns broken OpenPGP keys (ie. without a user id and thus without important information)

Thank you very much for yours answers, explanations and effort!!!

keys.openpgp.org has two problems: a) it is a centralized service due to the requirement to confirm mail addresses. b) For non-confirmed keys it returns broken OpenPGP keys (ie. without a user id and thus without important information). For these reasons and the general problems with the keyserver-(networks) there is no more default.

Windows7 has long reached end-of-life. Do not use it unless you have a fully air-gapped system. In this case, continue to use gpg4win 4.4.1 or resort to the command line of 5.0.0 which should still work.

Spent some time discovering and unfortunately it's Windows's bug in loopback interface.
I wrote a test demo (blocking mode) to exchange data and watched their packets, found that network stack would drop packets when congestion control algorithm is set to BBR2. It seems the second data exchange was broken.

Indeed, GnuPG's IPC uses TCP connections from 127.0.0.1 to 127.0.0.1 taking the destination port (and a cookie) from a file. We can't change that easily to the new Unix socket implementation Windows recently introduced. I hope there is a way to exclude localhost->localhost from congestion control.