Merging two mattermost server installations

I want to “merge” two mattermost server installations, as in taking all of the data (users, channels, posts, etc.) from one installation and migrate them into another existing installation.

In theory, this should be just a matter of dumping the database and data from the first installation, and then copying it over to the new installation. I’ve attempted this, and all data was transferred over successfully, but most users were not able to load the channels they had been a part of previously.

Steps to reproduce

  1. Dump SQL and data files from first server
  2. Insert all SQL data into second server, and copy data files over
  3. For any non-system-admin user from the first server, reset the password and sign in
  4. View team and channels, but fail to load any of the posts

Expected behavior
Users should be able to load the posts in the channels they had been in prior to the migration.

Observed behavior
Users can see all of their channels listed out in the web client, but trying to open any channel results in no posts being loaded. Checking the system console, trying to load posts results in a 403 forbidden response.

Presumably this is a permissions/roles issue with user accounts that came from the first server, but I’ve looked around at everywhere that permissions are specified and I can’t find anything that would indicate that those users should have any trouble viewing those posts.

Is there something I’m missing with how the permissions work?