I found the same issue with notes not being decrypted on export.
I got the solution here to work https://github.com/aviaryan/Evernote-Decrypt, though as Python 2.7 is deprecated and required for the script to work, that needed to be installed manually from here https://www.python.org/downloads/release/python-2718/
The encrypted notes were copied to a new notebook in EN, then exported as *.enex.
Ran the command line to decrypt.
Imported *.enex into Joplin.
Data was all intact.