A model-driven app using business rules has been working well in production for 2+ years. However, as of yesterday, the client-side (front-end) only works intermittently. I have replicated the issue in a simplified app, in a test environment, in order to isolate the problem.
Example:
1. I open the app (as sysadmin), and press CTRL+F5.
2. I open the entity view, and click on the New button.
3. Rule works well (fields shown are conditional based on the "Create Ticket By" field).

4. If I change the Create Ticket by field, the fields shown change accordingly

5. I save the record, and click New to create another record.
6. The business rule is no longer applied (all fields shown)

7. If I change the Create Ticket BY (which should show some fields, and hide others), nothing changes

Relaunching the app and refreshing the browser many times helps to make the business rules work once more... but it quickly stops working again as soon as a new record is once again created.
How can the erratic business rule behaviour be explained?