Schema type conversion in config yml when group role permissions are changed

Problem/Motivation

When I am altering permissions for a group role, and exporting the config, the schema type of permissions changes like this:

I am not sure what is causing this. The actual config changes gets lost in this diff, and it makes code review harder.

Not sure whether the problem is in Drupal core (I am using 8.6.16) or in this module.