We noticed that GnuPG 2.2.35 rejected some keys that were generated in our unit test. Surprisingly, only Windows seems to be affected: we tested GnuPG 2.2.35-2 from Debian testing, and it imports the key just fine.
Through manual testing we discovered that 2.2.34 is also affected, whereas 2.2.33 is fine. The tests use the binary distributions from https://gnupg.org/ftp/gcrypt/binary/
Because only some keys are rejected, this smells like an MPI encoding, and there is a change to that in the changelog, T5120. It is curious why this should only affect Windows though.
C:\Users\justus>gnupg-2.2.33\bin\gpg --import bad-sign-key.pgp gpg: keybox 'C:/Users/justus/AppData/Roaming/gnupg/pubring.kbx' created gpg: C:/Users/justus/AppData/Roaming/gnupg/trustdb.gpg: trustdb created gpg: key 8F73CF0A8EB36B6F: public key "someone@example.org" imported gpg: key 8F73CF0A8EB36B6F: secret key imported gpg: Total number processed: 1 gpg: imported: 1 gpg: secret keys read: 1 gpg: secret keys imported: 1 C:\Users\justus>rm -r AppData/Roaming/gnupg C:\Users\justus>rm -r AppData/Local/gnupg C:\Users\justus>gnupg-2.2.34\bin\gpg --import bad-sign-key.pgp gpg: keybox 'C:/Users/justus/AppData/Roaming/gnupg/pubring.kbx' created gpg: C:/Users/justus/AppData/Roaming/gnupg/trustdb.gpg: trustdb created gpg: key 8F73CF0A8EB36B6F: public key "someone@example.org" imported gpg: key 8F73CF0A8EB36B6F/8F73CF0A8EB36B6F: error sending to agent: Invalid argument gpg: key 8F73CF0A8EB36B6F/29C3C33E92C23B71: error sending to agent: Invalid argument gpg: error reading 'bad-sign-key.pgp': Invalid argument gpg: import from 'bad-sign-key.pgp' failed: Invalid argument gpg: Total number processed: 0 gpg: imported: 1 gpg: secret keys read: 1 C:\Users\justus>rm -r AppData/Roaming/gnupg C:\Users\justus>rm -r AppData/Local/gnupg C:\Users\justus>gnupg-2.2.35\bin\gpg --import bad-sign-key.pgp gpg: keybox 'C:/Users/justus/AppData/Roaming/gnupg/pubring.kbx' created gpg: C:/Users/justus/AppData/Roaming/gnupg/trustdb.gpg: trustdb created gpg: key 8F73CF0A8EB36B6F: public key "someone@example.org" imported gpg: key 8F73CF0A8EB36B6F/8F73CF0A8EB36B6F: error sending to agent: Invalid argument gpg: key 8F73CF0A8EB36B6F/29C3C33E92C23B71: error sending to agent: Invalid argument gpg: error reading 'bad-sign-key.pgp': Invalid argument gpg: import from 'bad-sign-key.pgp' failed: Invalid argument gpg: Total number processed: 0 gpg: imported: 1 gpg: secret keys read: 1 C:\Users\justus>cat bad-sign-key.pgp -----BEGIN PGP PRIVATE KEY BLOCK----- xVgEYqsJ8xYJKwYBBAHaRw8BAQdAfUB/E4q/4KXVCYOwdx/KRJ5dfUzuWhKQS92F uIaVMuEAAQCBqKT9nDUo6VcdJkU9euHtJnVyzK8r9RVZDpFpKFHr5Q94wsALBB8W CgB9BYJiqwnzAwsJBwkQj3PPCo6za29HFAAAAAAAHgAgc2FsdEBub3RhdGlvbnMu c2VxdW9pYS1wZ3Aub3JnsWbhoMETNIFiPfRvM64Q1+g7Cpjf/w/QT5cVSrq1cW4D FQoIApsBAh4BFiEEbr3AxOjW2vy6BFtuj3PPCo6za28AAFVOAP0ZVd7jchLC7P9d +Cu2rUJ3sDWSmlZdDqFAYRIP87Ka2AEAlDYC/9ooEC+NhG00xpO8EvGxpq0S9frJ aFkUqfBOaQ/NE3NvbWVvbmVAZXhhbXBsZS5vcmfCwA4EExYKAIAFgmKrCfMDCwkH CRCPc88KjrNrb0cUAAAAAAAeACBzYWx0QG5vdGF0aW9ucy5zZXF1b2lhLXBncC5v cmdOTl3Tzas+nFXSEQCscDTYrb02k5nGyUkxJB0OYhHVUwMVCggCmQECmwECHgEW IQRuvcDE6Nba/LoEW26Pc88KjrNrbwAAOwQA/3JJh5DP5HBY2yi1pNWvw45oqDl5 GH6BJWieDbevaGagAQDcRo9xM8d44PRIxbl/RiKAtVll9LufuJ0aRRmdxQaqCsdY BGKrCfMWCSsGAQQB2kcPAQEHQMRGCST173LcHXGM4rL4jc7GrT1ohOlp8bSAnt9r bCAEAAEAxcO+aAGmeAj5oX5oI4XWP/PdkopaXzZpP+I8Efj2lgMQTMLAvwQYFgoB MQWCYqsJ8wkQj3PPCo6za29HFAAAAAAAHgAgc2FsdEBub3RhdGlvbnMuc2VxdW9p YS1wZ3Aub3Jno3ks/10/XSnOV+cQlINOFse1UbCdEosHuo4nDb5HOZoCmwK+oAQZ FgoAbwWCYqsJ8wkQKcPDPpLCO3FHFAAAAAAAHgAgc2FsdEBub3RhdGlvbnMuc2Vx dW9pYS1wZ3Aub3JnsMBBdQlPZkZCAi9BDWTSpnxVL5wQSxVdkMjLoJw9RVEWIQQS PtMJ9bs65OYxKoYpw8M+ksI7cQAA3bQBAMHk5D+pg89YiB+AxNhYz1218zRViZUX 5fQjGcal6s1GAQCP0pmojI6vMXiWaOo1ATGxloqvoWj3V+qA5RtPbxmKBRYhBG69 wMTo1tr8ugRbbo9zzwqOs2tvAADgzgEA22TQn3B2cc5hKL/oIrCz1Dp1FkF4iGkI y0KLnG75ZUMA/RcO6tgrtzHBGpXwHN2U9S+z4qtdqN6sPHBJ9rhVRLwD =zzAE -----END PGP PRIVATE KEY BLOCK-----