Create Feature Profile doesn't work on the latest preview

[sarahelsaig]

Describe the bug

Looks like it creates a new feature profile but doesn't store it. (I've noticed this while validating Lombiq.Hosting.Tenants.FeaturesGuard in anticipation of OC 1.9.)

EDIT: Also forgot to mention that even if I add a feature profile from a setup recipe it doesn't show up in the Feature Profiles list.

To Reproduce

Steps to reproduce the behavior:

1. Set up a new site.
2. Enable Tenants and Tenant Feature Profiles features.
3. Go to Admin > Multi-Tenancy > Feature Profiles.
4. Click "Add Feature Profile" button.
5. Give it any name and copy the example rules from the text under the Rules field.
6. Click "Save and Continue".
7. Get redirected to a Not Found page and there are no error logs.
8. Go back to the Feature Profiles page and the list is empty.

Expected behavior

It should create and store a new Feature Profile.

Screenshots

Despite my initial guess, the JSON is correctly parsed:

[MikeAlhayek]

its is a serialization issue. Fixed by #15465