It seems that this is a bug that has been hanging around since 2017 (Bug 115538).
GPG Suite 2019.2 November 15th, 2019
gpg (GnuPG/MacGPG2) 2.2.17
/usr/local/bin/gpg -> /usr/local/MacGPG2/bin/gpg2
Firefox not installed (no NSS path under SETTINGS > Security)
Steps to Reproduce:
1. Open LibreOffice
2. Go to Settings, Encryption
PGP keys shown as option for default keys
No PGP keys available
This is indeed a known problem and you found the correct bug already, thus setting to DUPLICATE.
*** This bug has been marked as a duplicate of bug 115538 ***