Here is a minimal reproducible demo that shows the problem: https://gist.github.com/unclechu/347bbc386e0659242de80e8ff38b0ec6 (requires Nix to run). This demo also shows a trick how to overcome the problem.

Even when I define ReqBody with Optional mod:

ReqBody' '[Optional, Strict] '[JSON] SomeType

In the Server type SomeType stays the same, without Maybe wrap that I would also expect from QueryParam for instance.