When exporting a key with --export-secret-key-p12 errors are not communicated to the calling process.
E.g. I now have an error with one key that
2020-11-18 13:55:42 gpg-agent DBG: chan_29 <- EXPORT_KEY E9F57B52081406370E6F16F1696381FE3FEE9C56
2020-11-18 13:55:42 gpg-agent command 'EXPORT_KEY' failed: Unusable secret key
The stdout is empty, the return code is 0.
Alternatively you can reproduce it by entering the passphrase wrong three times.
In Kleo I'll check for empty output for now but this should be fixed and an error returned.