D6 to D8 migration "No Migrations found"

When trying to perform a migrate-upgrade from a Drupal 6.38 site to Drupal 8.3.2, a migration is shown as created and visible under “Migrations” on the new D8 site, however, when trying to perform a migration on said group, I am only given the output “No Migrations found”. When running ‘drush ms’ it does not show any current migrations at all, just outputs white space.
If I try to run migrate-upgrade on the same database, it tells me that “‘migration_group’ entity with ID ‘migrate_drupal_6’ already exists”.

I have cleared drush cache, removed the migration group and tried multiple times, even done a few clean installs and rollbacks on the server, and this issue still persists. I can’t see any type of pattern for when this occurs either, some days it will run through the migration just fine, and then this issue comes back again. Not even reverting the server to an earlier point in time when the migrations were working has fixed the issue, but sometimes it will just magically start working again.

Any thoughts on what could be the problem?

Drupal version: 

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