That's how it works. Paid Nextcloud storage capacity is probably Murena's main income (the first gigabyte is free), and the other one is phone sales. They fork reputable open-source solutions to make them even easier in /e/OS, but the forking introduces new bugs and complaints.
They seem to have introduced some incompatibility with their Seedvault fork:
https://doc.e.foundation/os/apps/backup-and-restore
Warning: Backup only works between /e/OS devices: it can only restore a backup it made itself, on a phone that already runs /e/OS — never data from Android, iOS, or any other system. It also only works with a Murena Workspace account, and needs the Passwords app to store its encryption key.
I guess if you skip that and install regular Seedvault, it would work normally.
Nextcloud is indeed self-hostable. Seedvault goes inside it, or elsewhere.