I'm investigating GnuPG v5 functionality, so modified GnuPG sources to always generate v5 keys. This works (including using generated keys with GnuPG 2.4.0), however attempt to export non-encrypted secret key fails, producing the following packet dump for RSA key:
# off=0 ctb=95 tag=5 hlen=3 plen=2921 :secret key packet: version 5, algo 1, created 1686657810, expires 0, pkbytes 391 pkey[0]: [3072 bits] pkey[1]: [17 bits] protbytes: 0 skbytes: 201210263 skey[2]: [MPI_NULL] skey[3]: [2685 bits] skey[4]: [MPI_NULL] skey[5]: [MPI_NULL] # off=2924 ctb=b4 tag=13 hlen=2 plen=6 :user ID packet: "v5_rsa"
For encrypted secret key export/import back works well.