I had the same problem after switching to the new style dynamicplaceholders after upgrading to Sitecore 9.0.2. You will have to upgrade the dynamicplaceholders For the entire content tree to the new style. That will fix the issue. For all new content, your code will take care.

Mostly Following the blog

https://jeroen-de-groot.com/2018/01/17/fix-invalid-dynamic-placeholders-after-upgrading-to-sitecore-9/

with some customization worked! Customization were mostly to refactor and provide a tool in out tool set instead of an admin page.

I had the same problem after switching to the new style dynamicplaceholders after upgrading to Sitecore 9.0.2. You will have to upgrade the dynamicplaceholders For the entire content tree to the new style. That will fix the issue. For all new content, your code will take care.

Mostly Following the blog

https://jeroen-de-groot.com/2018/01/17/fix-invalid-dynamic-placeholders-after-upgrading-to-sitecore-9/

with some customization worked! Customization were mostly to refactor and provide a tool in our tool set instead of an admin page.