Your review or CI apps aren't getting the specific add-on plans you've specified in your app.json.
This is due to a change in how we provision add-ons for Review and CI apps. Please see this Dev Center changelog item and this Dev Center article on review apps. When you specify an add-on in your app.json, we'll pick a plan optimized by the add-on partner for ephemeral apps. If they haven't set up a specific ephemeral plan, then we'll pick the default, which is usually the cheapest plan they offer, frequently free.
We are excited to offer Heroku Review and CI apps to allow you to test and review your code in the same environment as production. As they've gotten more popular, we've been working with add-on partners to ensure they can best support the higher churn of these temporary apps. This also allows add-on partners to create plans optimized for ephemeral use - quicker deployment, no backups, less partner-side instrumentation and logging - at a lower cost to you.
Some add-on providers may choose to provide low cost, non-free ephemeral plans. This is typically to help them recoup the costs of provisioning associated with the higher turnover of Review and CI apps. Add-on billing is always pro-rated to the second.
Please contact Heroku support if you're seeing build or application issues with the automatic ephemeral plan negotiation.