I've had the same experience when trying to migrate from Heroku Redis to Redis Cloud: my 'forked' database was empty.
I contacted Redis Labs Support and they said:
We currently do not support the --fork
option to migrate data from
Heroku Redis to Redis Cloud ... [but] you can directly import your data
into your Redis Cloud database by following the steps in this
document.
I followed the steps under "Import from a Redis server" using my Heroku Redis host, port and password and was able to populate a Redis Cloud db from an older Heroku Redis instance that was using Redis 5.0.
BUT I could not get the process to work for a more recent Heroku Redis instance on v6.2!
I contacted Redis Labs Support again and this time they said:
Import from Redis can accept only Open Source Redis URLs and not an
Heroku (cloud) database. You must export your data to AWS S3, Azure,
or Google Cloud storage and then import it to the Redis Enterprise
cloud.
In the end, I managed to copy (Dump and Restore) the contents of my v6.2 Heroku Redis to Redis Cloud using
nicStuff's redis-migrate gist (a slightly modified version)...
BUT I've only managed to get this working from a Heroku Redis Mini plan. Other plans (Premium-0, etc) require TLS and I'm not sure if it's possible to connect to these using redis-cli...