Checkout redirect does not work when payments are made via PayPal.

### Reproduction Steps

1. Install `LifterLMS` and `LifterLMS PayPal`. Make sure these are the only plugins installed and activated.
2. Activate the `Twenty Twenty-Three` theme.
3. Enable PayPal and set it test mode.
4. Create a test student.
5. Create page called `Upsell Page`.
6. Create a test course and add a paid access plan for that course. Setup a checkout redirect for that access plan so that it redirects to the `Upsell Page` after purchase.
7. Open the site in an incognito window and login as the test student, then purchase the test course.

### Expected Behavior
It should redirect to the `Upsell Page`.

### Actual Behavior
It does not redirect to the `Upsell Page`, as you can see in [this video](https://share.zight.com/Z4upyybl).

This issue has be recreated:
+ [x] Locally
+ [ ] On a staging site
+ [x] On a production website
+ [x] With only LifterLMS and a default theme