Why does resigning of third party RPM packages fail?

Issue

• Attempting to resign a third party RPM package results in invalid signature:

# rpm -Kv gskcrypt64-8.0.55.14.linux.x86_64.rpm 
gskcrypt64-8.0.55.14.linux.x86_64.rpm:
    Header V3 RSA/SHA256 Signature, key ID fbbfca9c: NOKEY
    Header SHA1 digest: OK (139b6d610c4a89190ed703052b595704aa56f53e)
    V3 RSA/SHA256 Signature, key ID fbbfca9c: NOKEY
    MD5 digest: OK (8ee06cbe4830ba470236ab13d2228470)

# rpm --resign gskcrypt64-8.0.55.14.linux.x86_64.rpm
Enter pass phrase: 
Pass phrase is good.
gskcrypt64-8.0.55.14.linux.x86_64.rpm:

# rpm -Kv gskcrypt64-8.0.55.14.linux.x86_64.rpm 
gskcrypt64-8.0.55.14.linux.x86_64.rpm:
    Header V4 DSA/SHA1 Signature, key ID f5300cbc: OK
    Verify signature: BAD PARAMETERS (268 0x1b28ce0 280 (nil) 0x1b25fb0)   <------
    Header SHA1 digest: OK (139b6d610c4a89190ed703052b595704aa56f53e)
    Header SHA1 digest: OK (139b6d610c4a89190ed703052b595704aa56f53e)
    Verify signature: BAD PARAMETERS (1002 0x1b2a320 280 (nil) 0x1b25fb0)  <------
    MD5 digest: OK (8ee06cbe4830ba470236ab13d2228470)
    MD5 digest: OK (8ee06cbe4830ba470236ab13d2228470)
    V4 DSA/SHA1 Signature, key ID f5300cbc: OK

• Attempting to delete the existing signature results in a duplicate digest entry:

# rpm -Kv /gskcrypt64-8.0.55.14.linux.x86_64.rpm 
/gskcrypt64-8.0.55.14.linux.x86_64.rpm:
    Header V3 RSA/SHA256 Signature, key ID fbbfca9c: NOKEY
    Header SHA1 digest: OK (139b6d610c4a89190ed703052b595704aa56f53e)
    V3 RSA/SHA256 Signature, key ID fbbfca9c: NOKEY
    MD5 digest: OK (8ee06cbe4830ba470236ab13d2228470)

# rpm --delsign /gskcrypt64-8.0.55.14.linux.x86_64.rpm 
/gskcrypt64-8.0.55.14.linux.x86_64.rpm:

# rpm -Kv /gskcrypt64-8.0.55.14.linux.x86_64.rpm 
/gskcrypt64-8.0.55.14.linux.x86_64.rpm:
    Header V3 RSA/SHA256 Signature, key ID fbbfca9c: NOKEY
    Header SHA1 digest: OK (139b6d610c4a89190ed703052b595704aa56f53e)
    Header SHA1 digest: OK (139b6d610c4a89190ed703052b595704aa56f53e)
    V3 RSA/SHA256 Signature, key ID fbbfca9c: NOKEY
    MD5 digest: OK (8ee06cbe4830ba470236ab13d2228470)
    MD5 digest: OK (8ee06cbe4830ba470236ab13d2228470)