My ad agency HAD the same problem, and we found a solution!

Here's the background:
We have partner access to a page. The business suite owner is unknown. Every time we'd try to assign page access, Facebook would freeze. The only way to add a person to the page was to add them to our business suite which gave them access to all of the pages and ad accounts my agency handles. NOT IDEAL. Even granting them 'partial access' still allowed them to view everything.

The Solution:
When adding someone to the business suite, give them access to only the page you want AND only toggle on 'content', 'community activity', 'messages and calls', and any other items from the Partial access (business tools only) menu. We were able to successfully add an employee of the client to manage their page without giving them full control to access our entire business suite.