Nextcloud version: 19.0.3
Operating system and version Ubuntu 18.04
Apache 2.4
PHP version: 7.3
The issue you are facing:
I do not see the fields to enter the recovery key for the serverside encryption under the default encryption module radio button.
i found a link that advised to run the following command on the server
php occ encryption:disable-master-key
After i did that I have some fields but different from what I saw in earlier versions.
Seems it is not (yet) documented, for now I will leave it this way.
What is advised when updates are made, I need to do some and hope it will not break things.