We had a similar split. There are pros and cons to this approach. Reporting into the Product org, keeps you within that structure and helps with relationship building, staying up to date with the strategy within the product org. It also helps with your own personal growth since as a product person you have product people to support and develop you.
Where it can fall apart is that your manager or CPO may not have visibility in what your doing and the challenges you are facing and overcoming, recognition could also become an issue.
It's important that you manage up and create the necessary forums (with your HoE supporting) to communicate to senior stakeholders, your platform as a product strategy, and product horizons.
Reporting directly in to the HoE may keep you focused on the challenges within platform engineering, but may isolate you from the product function and impact your growth. You could take a matrix management approach that way you can benefit from alignment with the HoE
Hope that helps